Phylogeny of the vertebrate skeleton.
The vertebrate skeleton is formed from the mesoderm and consists of 3 sections: the skeleton of the head (skull), the axial skeleton of the trunk (chord, spine and ribs), the skeleton of the limbs and their belts.
The main directions of evolution of the axial skeleton:
1. Replacement of the chord with the spine, cartilage tissue with bone.
2. Differentiation of the spine into sections (from two to five).
3. Increase in the number of vertebrae in departments.
4. Formation of the chest.
Cyclostomes and lower fish retain the chord throughout their lives, but they already have the beginnings of vertebrae (paired cartilaginous formations located above and below the chord): the upper arcs in cyclostomes, and the lower ones in fish.
In bony fish, vertebral bodies develop, spinous and transverse processes appear, and a canal of the spinal cord is formed. The spine consists of 2 sections: trunk and tail. In the trunk region there are ribs that freely end on the ventral side of the body.
Amphibians have 2 new departments: cervical and sacral, each of them contains one vertebra. There is a cartilaginous sternum. The ribs in tailed amphibians are of insignificant length and never reach the sternum; in tailless amphibians, ribs are absent.
In the spine of reptiles, the cervical region is distinguished, which contains 8-10 vertebrae, the thoracic, lumbar (in these regions - 22 vertebrae), the sacral - 2 and the caudal, which can have several dozen vertebrae. The first two cervical vertebrae have a special structure, resulting in greater head mobility. The last three cervical vertebrae each have a pair of ribs. The first five pairs of ribs of the lumbothoracic region join the cartilaginous sternum to form the rib cage.
In mammals, the spine consists of 5 sections. The cervical region has 7 vertebrae, the thoracic - from 9 to 24, the lumbar - from 2 to 9, the sacral - 4-10 or more, in the caudal region - very large variations. There is a reduction of the ribs in the cervical and lumbar regions. Sternum bone. 10 pairs of ribs reach the sternum, forming the chest.
Ontophylogenetically determined skeletal anomalies: additional ribs at the seventh cervical or at the first lumbar vertebra, splitting of the posterior arch of the vertebrae, nonunion of the spinous processes of the vertebrae ( Spinabifida), an increase in the number of sacral vertebrae, the presence of a tail, etc.
The vertebrate skull develops as a continuation of the axial skeleton ( brain department) and as a support for the respiratory and anterior digestive systems ( visceral region).
The main directions of the evolution of the skull:
1. Combining the visceral (facial) with the brain, increasing the volume of the brain.
2. Reducing the number of bones of the skull due to their fusion.
3. Replacement of a cartilaginous skull with a bone one.
4. Movable connection of the skull with the spine.
The origin of the axial skull is associated with the metamerism (segmentation) of the head. Its bookmark comes from two main departments: chordal- on the sides of the chord, which preserves the division into segments ( parachordalia), prechordal- ahead of the chord ( trabeculae).
The trabeculae and parachordalia grow and merge together to form the cranium from below and laterally. Olfactory and auditory capsules grow to it. The lateral walls are filled with orbital cartilages. The axial and visceral skull develop differently and are not related to each other at the early stages of phylogenesis and ontogenesis. The brain skull goes through three stages of development: membranous, cartilaginous and bone.
In cyclostomes, the roof of the brain skull is connective tissue (membranous), and the base is formed by cartilaginous tissue. The visceral skull is represented by the skeleton of the preoral funnel and the gill, which in lampreys consists of a row of seven cartilages.
In lower fish, the axial skull is cartilaginous (Figure 8). The back of the head appears. The visceral skull consists of 5-6 metamerically located cartilaginous arches that cover the anterior part of the digestive tube. The first arch, the largest, is called the jaw arch. It consists of the upper cartilage - palatine square, which forms the primary upper jaw. The lower cartilage, Meckel's cartilage, forms the primary lower jaw. The second branchial arch - hyoid (hyoid), consists of two upper hyomandibular cartilages and two lower - hyoids. The hyomandibular cartilage on each side fuses with the base of the cerebral skull, the hyoid connects to the Meckel's cartilage. Thus, the jaw arch is connected to the cerebral skull and this type of connection of the visceral and cerebral skull is called hyostyle.
Figure 8. Jaws (according to Romer, Parsons, 1992). A-B - modification of the first two pairs of gill arches in the jaw of fish; G - shark head skeleton: 1 - skull, 2 - olfactory capsule, 3 - auditory capsule, 4 - spine, 5 - palatine-square cartilage (upper jaw), 6 - Meckel's cartilage, 7 - hyomandibular, 8 - hyoid, 9 - splash (the first underdeveloped gill slit), 10 - the first complete gill slit: D - a transverse section of the shark in the head area.
Bony fish develop a secondary bony skull. It is partly composed of bones that develop from the cartilages of the primary skull, as well as integumentary bones that are adjacent to the primary skull. The roof of the brain skull consists of paired frontal, parietal and nasal bones. In the occipital region there are occipital bones. In the visceral skull, secondary jaws develop from the integumentary bones. The role of the upper jaw passes to the integumentary bones that develop in the upper lip, the lower jaw, and also to the integumentary bones that develop in the lower lip. On other visceral arches, the integumentary bones do not develop. The type of connection between the cerebral and visceral skull is hyostyle. The skull of all fish is fixedly connected to the spine.
The skull of terrestrial vertebrates changes mainly due to the loss of gill respiration. In amphibians, a lot of cartilage is still preserved in the brain skull, it becomes lighter than the skull of fish. Characteristic of all terrestrial vertebrates is the movable connection of the skull with the spine. The greatest changes occur in the visceral skull. Amphibians have functioning secondary jaws. The first, the jaw arch, is partially reduced. The palatine-square cartilage of the first jaw arch fuses with the base of the cerebral skull - this type of connection is called autostyle. In this regard, the hyomandibular cartilage of the hyoid arch loses its role as a suspension of the jaw arch. It is transformed into the auditory ossicle (column) located in the auditory capsule. The lower cartilage of the first gill arch - Meckel's cartilage - is partially reduced, and the rest of it is surrounded by integumentary bones. The hyoid (lower cartilage of the second arch) is transformed into the anterior horns of the hyoid bone. The remaining visceral arches (there are 6 in total in amphibians) are preserved in the form of the hyoid bone and in the form of laryngeal cartilages.
In reptiles, the skull of an adult animal ossifies. There are a large number of integumentary bones. The connection of the visceral and cerebral skull occurs due to the square bone (the ossified back of the reduced palatine square cartilage). The skull is autostyle. Jaws are secondary. Changes in other parts of the visceral arches are the same as in amphibians. In reptiles, a secondary hard palate and zygomatic arches are formed.
In mammals, there is a decrease in the number of bones as a result of their fusion and an increase in the volume of the brain skull. The roof of the skull is formed by the frontal and parietal bones, the temporal region is covered by the zygomatic arch. The secondary maxillae form the anterior lower part of the skull. The lower jaw consists of one bone and its process forms a joint with which it connects to the brain skull.
The rudiments of the palatine square and Meckel's cartilage are transformed, respectively, into the auditory ossicles - the anvil and the malleus. The upper section of the hyoid arch forms the stirrup, the lower section forms the hyoid apparatus. Parts of the 2nd and 3rd branchial arches form the thyroid cartilage of the larynx, the 4th and 5th arches are converted into the remaining cartilages of the larynx. In higher mammals, the volume of the brain skull increases significantly. In humans, the size of the facial skull is significantly reduced compared to the brain region, the cranium is rounded and smooth. The zygomatic arch is formed (synapsid type of skull).
Ontophylogenetically determined defects of the skull: an increase in the number of bone elements (each bone can consist of a large number of bones), nonunion of the hard palate - "cleft palate", frontal suture, the upper part of the occipital scales can be separated from the rest by a transverse suture; in the upper jaw there is an unpaired incisor bone characteristic of other mammals, one auditory bone, the absence of a chin protrusion, etc.
The main directions of evolution of the skeleton of the belts and the free limb:
1. From the skin (metapleural) folds of the lancelet to the paired fins of fish.
2. From the multi-beam fin of fish to the five-fingered limb.
3. Increased mobility of the connection of the limbs with the belts.
4. Reduction in the number of bones of the free limb and their enlargement by fusion.
The basis for the formation of the limbs of vertebrates are the skin folds on the sides of the body (metapleural), which are present in the lancelet and fish larvae.
Due to the change in function, the metapleural folds changed their structure. In fish, muscles and a skeleton appeared in them, in the form of a metameric series of cartilaginous rays that form the internal skeleton of the fins. In higher fish, the fin rays are bony. The primary anterior girdle is an arc (mostly bony) that covers the body from the sides and from the ventral side. The belt lies superficially, covered with several bones homologous to the scapula and coracoid of higher vertebrates. It serves only to connect the fins with the secondary belt. The secondary belt consists of a large paired bone, which is attached to the roof of the skull on the dorsal side, and is connected to each other on the ventral side. The posterior belt of the fish is poorly developed. It is represented by a small paired plate. In lobe-finned fish, the fins began to serve as a support when moving along the ground, and changes occurred in them that prepared them for transformation into a five-fingered limb of terrestrial vertebrates (Figure 9). The number of bone elements decreased, they became larger: the proximal section is one bone, the middle section is two bones, the distal section is radially located rays (7-12). The articulation of the skeleton of the free limb with the girdle of the limbs became mobile, which allowed the lobe-finned fish to use their fins as a support for the body when moving along the ground.
Figure 9. Pectoral fin of a lobe-finned fish and forepaw of an ancient amphibian (after Carroll, 1992). 1 - kleytrum, 2 - scapula, 3 - basal, corresponding to the humerus, 4 - basal, corresponding to the ulna, 5 - basal, corresponding to the radius, 6 - radials, 7 - clavicle.
The next stage of evolution is the replacement of a strong connection of skeletal elements with movable joints, a decrease in the number of rows in the wrist and the number of bones in a row in higher vertebrates, a significant lengthening of the proximal (shoulder, forearm) and distal sections (fingers), as well as shortening of the bones of the middle section.
The limb of terrestrial vertebrates is a complex lever that serves to move the animal on land. Limb belts (shoulder blades, crows, collarbones) have the form of an arc that covers the body from the sides and bottom (Figure 10). To attach a free limb, there is a recess on the shoulder blade, and the belts themselves become wider, which is associated with a significant development of the muscles of the limbs. In terrestrial vertebrates, the pelvic girdle consists of 3 paired bones: the ilium, ischium and pubis (Figure 11). The ischial bones are connected to the sacrum. All three bones form the acetabulum. The dorsal section of the belts is well developed, which contributes to their stronger strengthening.
Figure 10. Comparison of the girdles of the forelimbs of looped fishes (left) and amphibians (right) (after Kvashenko, 2014). 1 - kleytrum, 2 - scapula, 3 - clavicle, 4 - sternum, 5 - coracoid, 6 - presternum, 7 - retrosternum.
In humans, there are ontophylogenetically determined anomalies of the limb skeleton: flat feet, accessory bones of the wrist, tarsus, accessory fingers or toes (polydactyly), etc.
Figure 11. Development of the pelvic girdle in terrestrial vertebrates in connection with the reduction of the ribs (according to Kvashenko, 2014). 1 - whole, 2 - ribs, 3 - abdominal spinous processes, 4 - pelvic plate of fish, 5 - fossa of the hip joint, 6 - ilium, 7 - pubic bone, 8 - ischium, 9 - femur, 10 - sacral vertebra .
Lesson 24. MAMMALIAN SKELETON
Equipment and materials
- Skeleton of a rabbit, cat or rat (one for two students).
- Vertebrae from different parts of the body (one for two students).
- Fore and hind limbs with belts (one for two students).
- Skulls of insectivores, rodents, carnivores, ungulates (one for two students).
- Tables: 1) skeleton of a mammal; 2) the structure of the vertebrae from different parts of the body; 3) skull (side and bottom view); 4) the skeleton of the limbs and their belts.
Introductory remarks
The mammalian skeleton retains features typical of the amniote skeleton. It consists of the cerebral and visceral skulls, spine, chest, limb skeleton and their girdles. The spine has a well-defined dissection into five sections: cervical, thoracic, lumbar, sacral and caudal. In the cervical region, with rare exceptions, there are always seven vertebrae. The first two vertebrae - atlas and epistrophy - have the same structure as in reptiles and birds. The vertebrae of mammals of the platycoel type have flat articular surfaces with cartilaginous discs.
The skull is characterized by an enlargement of the braincase, a rather late fusion of a number of bones in ontogenesis with the formation of complex complexes, the connection of bones with sutures, and a strong development of ridges for attaching muscles. In connection with the significant development of the organ of smell, the ethmoid bone appears. There are two occipital condyles. The visceral skeleton undergoes further changes: three bones appear in the middle ear cavity: the stirrup, anvil, and malleus. In mammals, the tympanic bone. The lower jaw is represented by only one bone - the dentary. The jaws have teeth. Like amphibians, but not like reptiles and birds, there are carpal and ankle joints.
Scull
brain skull
Occipital department: occipital bone; occipital foramen; occipital condyles.
Sides of the skull: squamosal bones with zygomatic processes; zygomatic; maxillary; intermaxillary (anterior); lacrimal; oculocuneate; pterygoid bones.
Skull roof: parietal; interparietal; frontal; nasal bones.
Bottom of the skull: main wedge-shaped; anterior wedge-shaped; rocky; pterygoid; palatines; palatine processes of the maxillary bones; lattice labyrinths; coulter; drum bone; choanae; exit openings for nerves, blood vessels, and the Eustachian tube.
Visceral skull
Lower jaw: dentaries with coronal, articular and angular processes.
Spine
Sections of the spine: cervical; chest; lumbar; sacral and caudal.
The structure of the trunk platycoelous vertebra, atlas and epistrophy.
Rib cage: ribs true and false; sternum (handle and xiphoid process).
Limb belts
Shoulder girdle: shoulder blades, clavicle (no coracoids). Pelvic girdle: innominate bones (fused iliac, ischial and pubic bones).
Paired limbs
Forelimb: shoulder; forearm (radius and ulna); brush (wrist, metacarpus, phalanges of fingers).
Hind limb: hip; lower leg (large and small tibia); foot (tarsus, metatarsus, phalanges).
Sketch :
skull (side and bottom view).
Skeleton structure
The skull of mammals is relatively large, due to the increase in the size of the brain box (Fig. 119). The bones are heavy and thick, connected to each other by sutures. The eye sockets are relatively small. Groups of bones fuse into complexes, which include, in particular, the occipital and petrous bones.
In mammals, two new bones appear - the ethmoid (in the nasal cavity) and the interparietal (skull roof). A number of ancestral bones undergo both structural and functional changes, especially in the visceral skeleton. In the region of the middle ear, there are three auditory ossicles: the stirrup (formerly the hyomandibular, which first appeared in amphibians), the incus (the former quadrate bone), and the malleus (the former articular bone). The middle ear itself is covered by the tympanic bone (pair), which is characteristic only of mammals, and comes from the angular bone. Thus, the lower jaw of mammals is formed only by a pair of integumentary dentary connected directly to the brain skull.
Mammals have a well-developed secondary hard palate and a zygomatic arch peculiar only to them.
Rice. 119. Cat skull on the side ( A), bottom ( B) and her lower jaw ( V):
1
- occipital bone; 2
- occipital condyle, 3
- occipital foramen; 4
- parietal bone; 5
- interparietal bone; 6
- frontal bone; 7
- nasal bone; 8
- squamous bone; 9
- zygomatic process of the squamous bone; 10
- cheekbone; 11
- auditory drum; 12
- auditory opening; 13
- wing-sphenoid bone; 14
- oculocphenoid bone; 15
- main sphenoid bone 16
- anterior sphenoid bone; 17
- lacrimal bone; 18
- maxillary bone 19
- intermaxillary bone; 20
- palatine bone 21
- pterygoid bone; 22
- dental bone; 23
- coronoid process of the dentary; 24
- articular process of the dentary; 25
- angular process; 26
- stony bone
brain skull
Occipital region of the skull represented by a single occipital bone surrounding the foramen magnum. On the sides of it are two condyles that provide connection to the spine. The occipital bone is formed by the early fusion of four bones: the superior occipital, the two lateral occipitals, and the main occipital.
sides of the skull in the posterior part they are limited by squamosal bones with strongly developed zygomatic processes. The zygomatic process is directed forward and bears the articular surface for the lower jaw. It connects to the zygomatic bone, which, in turn, is attached to the zygomatic process of the maxillary bone. As a result, a zygomatic arch is formed, which is characteristic only for mammals. Behind the squamosal bone adjoins the stony bone (fused ear bones of the ancestors).
eye socket lined with pterygosphenoid, oculocphenoid and lacrimal bones. The oculosphenoid bone forms the interorbital septum. In the posterior corner of the orbit lies the pterygosphenoid
bone, and in the anterior - the lacrimal bone, penetrated by the lacrimal canal.
The ethmoid bone appears in the nasal cavity of mammals. Its middle part forms the nasal septum. The appearance of this bone is associated with the excellent development of the sense of smell in mammals.
skull roof formed by paired bones of skin origin: nasal, frontal and parietal. The latter in some mammals fuse into one bone. Between the parietal and occipital bones there is an interparietal bone, which is characteristic only of mammals. It can remain independent or fuse with neighboring bones.
Behind the bottom of the skull partly formed by the occipital bone. In front of it is the main sphenoid bone. In all amniotes, this bone is well developed. In front of it is the anterior sphenoid bone, protruding forward with a small wedge. In the back of the bottom of the skull, paired swellings are clearly visible - tympanic bones that cover the cavity of the middle ear. These bones are derived from the angular bone (visceral skeleton) of the ancestors. They open outward through the ear canal. The anterior part of the skull floor is represented by a secondary hard palate, characteristic of mammals, formed by the palatine bones and the palatine processes of the premaxillary and maxillary bones. Such a device allows the animal to breathe while chewing food.
Visceral skull
Visceral, or facial, skull mammals has characteristic features. The secondary maxilla, as in all higher vertebrates, fuses tightly with the cranium. The lower jaw is represented by only one bone - the dentary. This feature is a good marker for distinguishing the skull of mammals from the skulls of other vertebrates. The dentary has three processes: coronal, articular and angular. This bone bears the teeth. The articular process, with its convex surface, is connected to the zygomatic process of the squamous bone, on which there is an articular surface. Thus, there is a direct articulation of the lower jaw with the brain skull, bypassing the inserted elements of the visceral skeleton of all other vertebrates.
Maxillary and intermaxillary bones ( secondary upper jaw) in mammals, like in all amniotes, adhere to the brain skull, forming its anterior section. These bones carry teeth.
During embryonic development, mammals, like other vertebrates, develop the palatine-square and Meckel cartilages ( primary jaw arch). The posterior part of the palatine-square cartilage ossifies in the form of a square bone, which in all vertebrates, starting with bony fish, serves as the site of attachment of the lower jaw. In mammals, the square bone is transformed into the auditory bone - the anvil. Meckel's cartilage also ossifies. In bony fish, it is replaced by articular and angular bones. In mammals, the articular bone turns into another auditory bone - the malleus. The angular bone, as already mentioned, forms the tympanic bone.
Upper section hyoid arch- the hyomandibular, starting with amphibians, is transformed into the auditory ossicle - the stirrup. The lower part of the hyoid arch (hyoid and copula), as well as the first gill arch in mammals, are represented by the hyoid bone with anterior and posterior horns. The remaining elements of the gill arches are transformed into cartilages of the larynx.
Spine
The vertebral column of mammals is represented by five sections: cervical, thoracic, lumbar, sacral and caudal (Fig. 120). Vertebrae platycelial type, the surface of the vertebral body is flat. Between them are cartilaginous layers, or menisci.
For cervical characteristically constant number of vertebrae - seven. Thus, the length of the neck of mammals depends on the size of the vertebrae themselves, and not on their number. So, in a giraffe, a whale and a mole, the number of cervical vertebrae is the same. Only in the manatee (a detachment of sirens) and in sloths (a detachment of edentulous) the number of cervical vertebrae is different (6 - 10).
The first two cervical vertebrae in mammals, like all amniotes, are transformed. The annular atlas rotates around its own body - the odontoid process, which is attached to the body of the second vertebra - the epistrophy (Fig. 121). Atlas bears two articular surfaces for connection with the condyles of the skull.
The rest of the vertebrae have a typical structure (Fig. 122). Each vertebra consists of a body, superior arch with superior spinous process, and transverse processes. The vertebrae have cartilaginous articular surfaces for movable connection with each other.
V thoracic region the number of vertebrae varies from 9 to 24, although usually 12 - 13. The spinous processes of the vertebrae are large,
Rice. 120. Rabbit Skeleton:
1
- cervical vertebrae; 2
- thoracic vertebrae; 3
- lumbar vertebrae; 4
- sacrum; 5
- tail vertebrae; 6
- ribs; 7
- handle of the sternum; 8
- scapula; 9
- acromial process of the scapula; 10
- coracoid process of the scapula; 11
- iliac part of the innominate bone; 12
- ischium of the innominate bone; 13
- pubic part of the innominate bone; 14
- obturator opening; 15
- brachial bone; 16
- elbow bone; 17
- radius bone; 18
- wrist; 19
- metacarpus; 20
- hip; 21
- knee cap; 22
- tibia; 23
- small tibia; 24
- heel bone; 25
- other bones of the tarsus; 26
- metatarsus; 27
- olecranon
directed backwards. Ribs are attached to thick and short transverse processes.
Vertebrae lumbar massive, do not bear ribs (they are rudimentary). Their number varies in different species from 2 to 9. Their spinous processes are small, directed forward towards those of the thoracic vertebrae.
Rice. 121. The first two cervical vertebrae of a mammal:
A- atlas; B- epistrophy (top and side); 1
- transverse process; 2
- odontoid process; 3
- superior spinous process
Rice. 122. The structure of the thoracic vertebra of a cat from the side ( A) and in front ( B):
1
- vertebral body; 2
- upper arch; 3
- superior spinous process; 4
- transverse processes
sacral the vertebrae fuse together to form the sacrum. A powerful sacrum helps to strengthen the connection through the belt of the hind limbs with the axial skeleton. The number of sacral vertebrae is usually 2 - 4, although it can reach 10 (in edentulous). Moreover, there are usually 2 true sacral, the rest are initially caudal.
Tail vertebrae have shortened processes. The number of tail vertebrae varies from 3 (gibbon) to 49 (long-tailed pangolin). It is interesting to note that some great apes have fewer tail vertebrae than humans. For example, an orangutan has 3 of them, a person has 3 - 6 (usually 4).
Rib cage
The thorax of mammals is formed by the sternum and ribs, attached at one end to the sternum, and at the other - to the transverse processes of the thoracic vertebrae. Sternum- segmented plate, consisting of the upper part - the handle - and the lower part - the xiphoid process. Ribs subdivided into true, which articulate with the sternum (there are usually seven in mammals), and false, which do not reach the sternum.
Limb belts
Shoulder girdle of all tetrapods is normally formed by paired bones: scapula, coracoid and clavicle. In mammals, not all elements of the shoulder girdle of terrestrial vertebrates are developed (Fig. 123).
The scapula is represented by a wide triangular bone lying on top of the chest. It has a well-marked ridge ending in the acromial process. The comb serves to attach the muscles.
The coracoid is found only in oviparous mammals. The rest
Rice. 123. Shoulder girdle and forelimb of a fox:
1
- scapula; 2
- comb of the scapula; 3
- acromial process; 4
- articular fossa; 5
- coracoid process; 6
- brachial bone; 7
- elbow bone; 8
- radius bone; 9
- wrist; 10
- metacarpus; 11
- phalanges of fingers
(real animals) coracoid in the form of a separate bone exists only in the embryonic state. During ontogenesis, it adheres to the scapula, forming a coracoid process. This process is directed forward and somewhat hangs over the humerus.
The clavicle is represented by a rod-shaped bone that connects the scapula to the sternum. The clavicle not only strengthens the articular fossa, attaching the shoulder girdle to the chest, but also allows the forelimb to move in different planes in many animals (for example, moles, monkeys, bats, bears). In fast running and jumping mammals, whose forelimbs move in the same plane (forward - backward), the clavicle is reduced. So, it is absent in ungulates, some carnivores, proboscis. In these animals, the shoulder girdle (more precisely, the shoulder blade) is connected to the axial skeleton only by ligaments and muscles.
Pelvic girdle mammals (Fig. 124) is typical for tetrapods. It is represented by paired nameless bones, which were formed as a result of the fusion of three pairs of bones: the ilium, ischium and pubis. The iliac regions of the innominate, as usual, point upwards and are connected to the sacral vertebrae (sacrum); ischial - go down and back; pubic - down and forward. Below, the innominate bones fuse to form the symphysis. Thus, the pelvis in mammals, like in reptiles, is closed. In the lower part of the innominate bone there is an obturator foramen. At the junction point of all three sections of the pelvic girdle, the acetabulum is formed - the place of articulation of the hind limb. In cloacae and marsupials, skin marsupials adjoin the pubic region.
Paired limbs
The skeleton of the paired limbs of mammals has all the typical features of the original five-fingered tetrapod limb. It is a complex lever, consisting of three departments. In the forelimb, these are the shoulder, forearm and hand; in the back - thigh, lower leg and foot. The joints between the lower leg and foot (ankle), as well as the forearm and hand (forearm-carpal) are of the "amphibian" type, in contrast to reptiles and birds, in which these joints are formed, respectively, between the bones of the metatarsus and the bones of the wrist.
In the forelimb, the shoulder is formed by the humerus (see Fig. 123). The forearm consists of the radius and ulna. The radius goes in the direction of the first (inner) finger. The ulna is directed towards the last (outer) finger. In the upper part, it has an olecranon. The hand, in turn, is formed by three sections: the wrist, metacarpus and phalanges of the fingers. The wrist consists of 8 - 10 bones arranged in 3 rows. There are five bones in the metacarpus, the same number of fingers. The fingers usually have three phalanges, with the exception of the first, which has two phalanges.
The hind limb of mammals (see Fig. 124) consists of three sections: thigh, lower leg and foot. The thigh is represented by a massive elongated femur. The lower leg is formed by two bones - the tibia and the tibia. They are the same length, but differ in thickness and position. The large tibia occupies an internal position and is directed towards the first finger. The fibula is located outside and approaches the last (outer) finger. The joint between the thigh and lower leg is covered in front by the patella characteristic of mammals, formed from ossified muscle tendons. The foot is represented by three rows of tarsal bones. Among them, the heel bone stands out. There are five bones in the metatarsus. Fingers are attached to them. The fingers usually have three phalanges, with the exception of the thumb (inner) finger, where most often there are two phalanges.
In connection with the existence of mammals in various conditions and their adaptation to various types of movement, the described type of limbs in some representatives undergoes changes. In all animals, the nature of the movement of which is associated with fast running or jumping, one bone remains in the lower leg, and often in the forearm, respectively, the tibia and ulna (ungulates, canines, kangaroos, jerboas, etc.). In addition, they are characterized by the appearance of an additional
leverage and shock absorber: the metatarsal bones lengthen and merge into one. In good runners, the number of fingers is reduced from five to four (artiodactyls) and even to one (equids). In artiodactyls, fingers III and IV are predominantly developed, in equids - III. In bats, the phalanges II - V of the fingers of the forepaws are elongated, a leathery membrane of the wing is stretched between them. Among mammals there are plantigrades (bears, hedgehogs, moles, monkeys) and digitigrades (ungulates, canines).
In the process of evolution, animals mastered more and more new territories, types of food, adapted to the changed living conditions. Evolution gradually changed the appearance of animals. In order to survive, it was necessary to actively search for food, better hide or defend against enemies, and move faster. Changing along with the body, the musculoskeletal system had to provide all these evolutionary changes. The most primitive protozoa do not have supporting structures, move slowly, flowing with the help of pseudopods and constantly changing shape.
The first support structure that appeared - cell membrane. It not only delimited the organism from the external environment, but also made it possible to increase the speed of movement due to flagella and cilia. Multicellular animals have a wide variety of supporting structures and adaptations for movement. Appearance external skeleton increased the speed of movement due to the development of specialized muscle groups. Internal skeleton grows with the animal and allows you to reach record speeds. All chordates have an internal skeleton. Despite significant differences in the structure of the musculoskeletal structures in different animals, their skeletons perform similar functions: support, protection of internal organs, and movement of the body in space. The movements of vertebrates are carried out by the muscles of the limbs, which carry out such types of movement as running, jumping, swimming, flying, climbing, etc.
Skeleton and muscles
The musculoskeletal system is represented by bones, muscles, tendons, ligaments and other connective tissue elements. The skeleton determines the shape of the body and, together with the muscles, protects the internal organs from all kinds of damage. Thanks to the connections, the bones can move relative to each other. The movement of bones occurs as a result of the contraction of the muscles that attach to them. In this case, the skeleton is a passive part of the motor apparatus that performs a mechanical function. The skeleton consists of dense tissues and protects the internal organs and the brain, forming natural bone containers for them.
In addition to mechanical functions, the skeletal system performs a number of biological functions. The bones contain the main supply of minerals that are used by the body as needed. The bones contain red bone marrow, which produces blood cells.
The human skeleton consists of a total of 206 bones - 85 paired and 36 unpaired.
The structure of the bones
The chemical composition of bones
All bones are composed of organic and inorganic (mineral) substances and water, the mass of which reaches 20% of the bone mass. Organic matter of bones ossein- has elastic properties and gives the bones elasticity. Minerals - salts of carbonate, calcium phosphate - give the bones hardness. The high strength of bones is provided by a combination of the elasticity of ossein and the hardness of the mineral substance of the bone tissue.
Macroscopic structure of the bone
Outside, all bones are covered with a thin and dense film of connective tissue - periosteum. Only the heads of the long bones do not have a periosteum, but are covered with cartilage. The periosteum contains many blood vessels and nerves. It provides nutrition to the bone tissue and takes part in the growth of the bone in thickness. Thanks to the periosteum, broken bones grow together.
Different bones have a different structure. A long bone has the appearance of a tube, the walls of which consist of a dense substance. Such tubular structure long bones gives them strength and lightness. In the cavities of tubular bones is yellow bone marrow- Loose connective tissue rich in fat.
The ends of long bones contain cancellous bone. It also consists of bony plates that form many crossed partitions. In places where the bone is subjected to the greatest mechanical load, the number of these partitions is the highest. In the spongy substance is red bone marrow whose cells give rise to blood cells. Short and flat bones also have a spongy structure, only from the outside they are covered with a layer of dam substance. The spongy structure gives the bones strength and lightness.
Microscopic structure of the bone
Bone tissue refers to the connective tissue and has a lot of intercellular substance, consisting of ossein and mineral salts.
This substance forms bony plates arranged concentrically around microscopic tubules that run along the bone and contain blood vessels and nerves. Bone cells, and therefore bone, are living tissue; it receives nutrients from the blood, metabolism takes place in it and structural changes can occur.
Bone types
The structure of bones is determined by the process of long historical development, during which the body of our ancestors changed under the influence of the environment and adapted by natural selection to the conditions of existence.
Depending on the shape, there are tubular, spongy, flat and mixed bones.
tubular bones are found in organs that make rapid and extensive movements. Among the tubular bones there are long bones (humerus, femur) and short ones (phalanxes of fingers).
In tubular bones, a middle part is distinguished - the body and two ends - the heads. Inside the long tubular bones there is a cavity filled with yellow bone marrow. The tubular structure determines the strength of the bones necessary for the body while consuming the least amount of material for them. During the period of bone growth, cartilage is located between the body and the head of the tubular bones, due to which the bone grows in length.
flat bones limit cavities inside which organs are placed (cranial bones), or serve as surfaces for attachment of muscles (scapula). Flat bones, like short tubular bones, are predominantly spongy. The ends of long tubular bones, as well as short tubular and flat bones, do not have cavities.
spongy bones built mainly of spongy substance, covered with a thin layer of compact. Among them, long spongy bones (sternum, ribs) and short ones (vertebrae, wrist, tarsus) are distinguished.
TO mixed bones include bones that are composed of several parts that have a different structure and function (temporal bone).
Protrusions, ridges, roughness on the bone - these are the places of attachment to the bones of the muscle. The better they are expressed, the stronger the muscles attached to the bones are developed.
Human skeleton.
The skeleton of man and most mammals has the same type of structure, consists of the same sections and bones. But man differs from all animals in his ability to work and intellect. This left a significant imprint on the structure of the skeleton. In particular, the volume of the human cranial cavity is much larger than that of any animal that has a body of the same size. The size of the facial part of the human skull is smaller than that of the brain, while in animals, on the contrary, it is much larger. This is due to the fact that in animals the jaws are an organ of protection and obtaining food and therefore are well developed, and the volume of the brain is smaller than in humans.
The curvatures of the spine associated with the shift of the center of gravity due to the vertical position of the body contribute to maintaining a person's balance and soften shocks. Animals do not have such curves.
The human chest is compressed from front to back and close to the spine. In animals, it is compressed from the sides and extended to the bottom.
The wide and massive human pelvic girdle looks like a bowl, supports the abdominal organs and transfers body weight to the lower limbs. In animals, body weight is evenly distributed between the four limbs and the pelvic girdle is long and narrow.
The bones of the lower extremities of a person are noticeably thicker than the upper ones. Animals do not have a significant difference in the structure of the bones of the fore and hind limbs. The great mobility of the forelimbs, especially the fingers, makes it possible for a person to perform various movements and types of work with his hands.
Torso skeleton axial skeleton
Torso skeleton includes the spine, consisting of five sections, and the thoracic vertebrae, ribs and sternum form chest(see table).
Scull
In the skull, brain and facial sections are distinguished. V cerebral part of the skull - the cranium - is the brain, it protects the brain from shock, etc. The cranium consists of fixedly connected flat bones: the frontal, two parietal, two temporal, occipital and main. The occipital bone connects to the first vertebrae of the spine with the help of an elliptical joint, which ensures that the head tilts forward and to the side. The head rotates along with the first cervical vertebra due to the connection between the first and second cervical vertebrae. There is a hole in the occipital bone through which the brain connects to the spinal cord. The bottom of the cranium is formed by the main bone with numerous openings for nerves and blood vessels.
Facial the skull section forms six paired bones - the upper jaw, zygomatic, nasal, palatine, lower nasal concha, as well as three unpaired bones - the lower jaw, vomer and hyoid bone. The mandibular bone is the only bone of the skull that is movably connected to the temporal bones. All bones of the skull (with the exception of the lower jaw) are fixedly connected, which is due to the protective function.
The structure of the facial skull in humans is determined by the process of "humanization" of the monkey, i.e. the leading role of labor, the partial transfer of the grasping function from the jaws to the hands, which have become organs of labor, the development of articulate speech, the use of artificially prepared food, which facilitates the work of the chewing apparatus. The brain skull develops in parallel with the development of the brain and sensory organs. In connection with the increase in the volume of the brain, the volume of the cranium has increased: in humans, it is about 1500 cm 2.
Torso skeleton
The skeleton of the body consists of the spine and chest. Spine- the basis of the skeleton. It consists of 33-34 vertebrae, between which there are cartilaginous pads - disks, which gives the spine flexibility.
The human spinal column forms four bends. In the cervical and lumbar spine, they bulge forward, in the thoracic and sacral - back. In the individual development of a person, bends appear gradually, in a newborn the spine is almost straight. First, a cervical bend is formed (when the child begins to hold his head straight), then the chest (when the child begins to sit). The appearance of the lumbar and sacral curves is associated with maintaining balance in the vertical position of the body (when the child begins to stand and walk). These bends are of great physiological importance - they increase the size of the chest and pelvic cavities; make it easier for the body to maintain balance; soften shocks when walking, jumping, running.
With the help of intervertebral cartilage and ligaments, the spine forms a flexible and elastic column with mobility. It is not the same in different parts of the spine. The cervical and lumbar sections of the spine have greater mobility, the thoracic section is less mobile, as it is connected to the ribs. The sacrum is completely immobile.
Five sections are distinguished in the spine (see the diagram "Departments of the spine"). The size of the vertebral bodies increases from the cervical to the lumbar due to the greater load on the underlying vertebrae. Each of the vertebrae consists of a body, a bony arch, and several processes to which muscles are attached. There is a hole between the vertebral body and the arch. The openings of all vertebrae form spinal canal in which the spinal cord is located.
Rib cage formed by the sternum, twelve pairs of ribs and thoracic vertebrae. It serves as a container for important internal organs: the heart, lungs, trachea, esophagus, large vessels and nerves. Takes part in respiratory movements due to the rhythmic raising and lowering of the ribs.
In humans, in connection with the transition to upright posture, the hand is also freed from the function of movement and becomes an organ of labor, as a result of which the chest experiences traction from the attached muscles of the upper limbs; the insides do not press on the front wall, but on the lower one, formed by the diaphragm. This causes the chest to become flat and wide.
Skeleton of the upper limb
Upper limb skeleton consists of a shoulder girdle (scapula and collarbone) and a free upper limb. The shoulder blade is a flat triangular bone adjacent to the back of the chest. The clavicle has a curved shape, resembling the Latin letter S. Its significance in the human body lies in the fact that it puts the shoulder joint at some distance from the chest, providing greater freedom of movement of the limb.
The bones of the free upper limb include the humerus, the bones of the forearm (radius and ulna) and the bones of the hand (the bones of the wrist, the bones of the metacarpus and the phalanges of the fingers).
The forearm is represented by two bones - the ulna and the radius. Due to this, it is capable of not only flexion and extension, but also pronation - turning in and out. The ulna in the upper part of the forearm has a notch that connects to the block of the humerus. The radius connects to the head of the humerus. In the lower part, the radius has the most massive end. It is she who, with the help of the articular surface, together with the bones of the wrist, takes part in the formation of the wrist joint. On the contrary, the end of the ulna here is thin, it has a lateral articular surface, with the help of which it connects to the radius and can rotate around it.
The hand is the distal part of the upper limb, the skeleton of which is the bones of the wrist, metacarpus and phalanx. The wrist consists of eight short spongy bones arranged in two rows, four in each row.
skeleton hand
Hand- the upper or forelimb of man and monkeys, for which the ability to oppose the thumb to everyone else was previously considered a characteristic feature.
The anatomical structure of the hand is quite simple. The arm is attached to the body through the bones of the shoulder girdle, joints and muscles. Consists of 3 parts: shoulder, forearm and hand. The shoulder girdle is the most powerful. Bending the arms at the elbow gives the arms greater mobility, increasing their amplitude and functionality. The hand consists of many movable joints, it is thanks to them that a person can click on the keyboard of a computer or mobile phone, point a finger in the right direction, carry a bag, draw, etc.
The shoulders and hands are connected by means of the humerus, ulna and radius bones. All three bones are connected to each other with the help of joints. At the elbow joint, the arm can be bent and extended. Both bones of the forearm are connected movably, therefore, during movement in the joints, the radius rotates around the ulna. The brush can be rotated 180 degrees.
Skeleton of the lower extremities
Skeleton of the lower limb consists of a pelvic girdle and a free lower limb. The pelvic girdle consists of two pelvic bones articulated behind the sacrum. The pelvic bone is formed by the fusion of three bones: the ilium, ischium, and pubis. The complex structure of this bone is due to a number of functions it performs. Connecting with the hip and sacrum, transferring the weight of the body to the lower limbs, it performs the function of movement and support, as well as a protective function. In connection with the vertical position of the human body, the pelvic skeleton is relatively wider and more massive than in animals, since it supports the organs lying above it.
The bones of the free lower limb include the femur, lower leg (tibia and fibula), and foot.
The skeleton of the foot is formed by the bones of the tarsus, metatarsus and phalanges of the fingers. The human foot differs from the animal foot in its vaulted shape. The vault softens the shocks received by the body when walking. The toes are poorly developed in the foot, with the exception of the big one, since it has lost its grasping function. The tarsus, on the contrary, is strongly developed, the calcaneus is especially large in it. All these features of the foot are closely related to the vertical position of the human body.
The upright posture of a person has led to the fact that the difference in the structure of the upper and lower extremities has become much greater. Human legs are much longer than arms, and their bones are more massive.
Bone joints
In the human skeleton, there are three types of bone connections: fixed, semi-movable and movable. Fixed the type of connection is the connection due to the fusion of bones (pelvic bones) or the formation of sutures (skull bones). This fusion is an adaptation to bear the heavy load experienced by the human sacrum due to the vertical position of the torso.
semi-movable connection is made with cartilage. The bodies of the vertebrae are interconnected in this way, which contributes to the inclination of the spine in different directions; ribs with a sternum, which ensures the movement of the chest during breathing.
Movable connection, or joint, is the most common and at the same time complex form of bone connection. The end of one of the bones that form the joint is convex (the head of the joint), and the end of the other is concave (the articular cavity). The shape of the head and cavity correspond to each other and the movements carried out in the joint.
articular surface articulating bones are covered with white shiny articular cartilage. The smooth surface of the articular cartilage facilitates movement, and its elasticity softens the jolts and jolts experienced by the joint. Usually, the articular surface of one bone that forms the joint is convex and is called the head, while the other is concave and is called the cavity. Due to this, the connecting bones fit tightly to each other.
Articular bag stretched between the articulating bones, forming a hermetically closed joint cavity. The articular bag consists of two layers. The outer layer passes into the periosteum, the inner one secretes a fluid into the joint cavity, which plays the role of a lubricant, ensuring the free sliding of the articular surfaces.
Features of the human skeleton associated with labor activity and upright posture
Labor activity
The body of a modern person is well adapted to labor activity and upright posture. Bipedal locomotion is an adaptation to the most important feature of human life - work. It is he who draws a sharp line between man and higher animals. Labor had a direct impact on the structure and function of the hand, which began to influence the rest of the body. The initial development of upright walking and the emergence of labor activity led to a further change in the entire human body. The leading role of labor contributed to the partial transfer of the grasping function from the jaws to the hands (which later became labor organs), the development of human speech, the use of artificially prepared food (facilitates the work of the chewing apparatus). The brain part of the skull develops in parallel with the development of the brain and sensory organs. In this regard, the volume of the cranium increases (in humans - 1,500 cm 3, in great apes - 400–500 cm 3).
bipedalism
A significant part of the signs inherent in the human skeleton is associated with the development of a bipedal gait:
- supporting foot with a strongly developed, powerful thumb;
- brush with a very developed thumb;
- the shape of the spine with its four curves.
The shape of the spine has developed due to a springy adaptation to walking on two legs, which ensures smooth movements of the body, protects it from damage during sudden movements and jumps. The trunk is flattened in the thoracic region, which leads to compression of the chest from front to back. The lower limbs have also undergone changes due to upright posture - widely spaced hip joints give stability to the body. In the course of evolution, the gravity of the body was redistributed: the center of gravity moved down and took a position at the level of 2–3 sacral vertebrae. A person has a very wide pelvis, and his legs are widely spaced, this makes it possible for the body to be stable when moving and standing.
In addition to the spine with a curved shape, five vertebrae in the sacrum, compressed chest, one can note the elongation of the scapula and the expanded pelvis. All this resulted in:
- strong development of the pelvis in width;
- fastening of the pelvis with the sacrum;
- powerful development and a special way to strengthen the muscles and ligaments in the hip area.
The transition of human ancestors to upright walking led to the development of the proportions of the human body, which distinguish it from monkeys. So for a person shorter upper limbs are characteristic.
Walking and labor led to the formation of asymmetry of the human body. The right and left halves of the human body are not symmetrical in shape and structure. A prime example of this is the human hand. Most people are right-handed, with about 2-5% left-handers.
The development of bipedalism, accompanying the transition of our ancestors to living in open areas, led to significant changes in the skeleton and the whole organism as a whole.
Phylogeny of the body cover. Starting from the lower chordates, a division of the outer integument or skin into a superficial epithelial layer of ectodermal origin (epidermis) and an underlying connective tissue layer developing from the mesoderm (corium or skin proper) is found. In the lancelet, the integumentary tissues are poorly developed, the epithelium is single-layer, cylindrical, contains separate glandular cells . The corium is represented by an insignificant layer of gelatinous connective tissue.
In the Vertebrate subtype, skin differentiation continues into distinct epidermis and corium. The epidermis becomes multi-layered, its lower layer consists of cylindrical cells that actively multiply and replenish the surface layers of cells. The corium is represented by the ground substance, fibers and cells. The skin forms a number of appendages, the main of which are protective formations and glands.
Fishes. In cartilaginous fish, the epidermis contains a large number of unicellular mucous glands. ("The corium is dense, fibrous. The whole body is covered with placoid scales, which are plates bearing a spike or a tooth. Its base lies in the corium, and the spike pierces the epidermis and comes out. The scale consists of dentin - a compound of organic matter with lime, harder, than bone, and does not contain cells.
The anlage of the placoid scale is formed at the border of the epidermis and corium. The lower layer of the epidermis takes the form of a cap, into which a mass of mesodermal cells is introduced in the form of a papilla. The cells that form the walls of the cap become cylindrical. The underlying cells of the mesoderm (scleroblasts) are also arranged in an orderly, continuous layer. The cells of this layer form
dentin plate - the base of the scale, covering the meso-dermal papilla. Randomly located in the middle of the cells form the pulp. Further thickening of the dentin occurs due to the layer of scleroblasts, on the surface of which new layers of dentin arise, due to which the spike grows and passes through the epidermis. Outside, the spike is covered with enamel, even harder than dentin.
At bony fish the body is also covered with scales, but unlike cartilaginous fish, it is bony. The scales have the form of rounded thin plates, overlapping each other in a tile-like manner and externally covered with a thin layer of the epidermis. The development of the bony scales proceeds entirely at the expense of the corium, without the participation of the epidermis. Phylogenetically, the bony scale is related to the more primitive placoid scale.
Amphibians. The skin of amphibians is naked, devoid of scales. The keratinization of the upper layer is weakly expressed. The corium is represented by connective tissue fibers running strictly parallel and cellular elements. There are many mucous glands in the skin. The skin glands create a liquid film on the surface, which promotes gas exchange (skin respiration) and protects the skin from drying out, since weak keratinization does not protect amphibians from water loss. In addition, the bactericidal properties of the secretion of the glands prevent the penetration of microbes. Poison glands protect the animal from enemies.
Reptiles. In connection with the transition to a terrestrial way of life in reptiles, the degree of keratinization of the epidermis (protection from drying out and from damage) increases. The scales become horny. The epidermis is clearly divided into two layers: the lower one (Malpighian), whose cells multiply intensively, and the upper one (horny), containing cells that gradually die off as a result of a special kind of degeneration. Drops of keratohyalin, a horny substance, appear in the cells, the amount of which gradually increases, the nucleus disappears, the cell flattens and turns into a hard horny scale, which then sloughs off. Due to the multiplication of cells of the alpighian layer, the cells of the stratum corneum are constantly replenished. The development of the horny scales at first proceeds in the same way as the bone. Differences in development are observed at the final stage and consist in the transformation of the epidermis. Reptiles lack skin glands.
Mammals. The skin of mammals has a particularly complex structure. Both layers - epidermis and corium are well developed. The epidermis gives rise to many derivatives of the skin - hair, nails, claws, hooves, horns, scales, various glands. The skin itself acquires a considerable thickness and consists mainly of fibrous connective tissue. In the lower part of the corium, a layer of subcutaneous adipose tissue is formed.
A characteristic feature of mammals is hair, the main function of which is to protect the body from heat loss. Hair is a horny appendage of a complex structure. In an adult, hair is present on the entire body, except for the palms and soles, but is greatly reduced.
The skin contains a large number of multicellular glands - sweat, sebaceous and milk. The sweat glands of mammals are homologous to the skin glands of amphibians. Sometimes sweat glands form local accumulations. The secretion of sweat glands, as a rule, has a liquid consistency and may be mucous or proteinaceous in composition, or contain fat. Sweat glands play an important role in the processes of excretion and thermoregulation. Evaporation of sweat is associated with a large loss of heat.
The sebaceous glands secrete a secret that lubricates the hair and skin surface, protecting it from environmental influences. The appearance of sebaceous glands is a hallmark of mammals.
The mammary glands are homologous to the sweat glands. The mammary glands of cloacal mammals (echidna, platypus) have the closest resemblance to the sweat glands, in which they are located in a group on the so-called glandular field, which is located in the bag for bearing eggs and cubs. The secret flows to the surface and is licked off by the young. Marsupials have a nipple, where each gland opens with its own opening. Along the edges of the developing nipple, all successive transitions between the normal sweat and typical mammary glands can be found.
In viviparous, a paired strip of thickened epithelium is laid on the sides of the belly - the milky line, and on it are the mammary glands and nipples.
The main direction of the evolution of the outer covers is the differentiation of the layers of the skin and its derivatives (glands, scales, feathers, hair), which provide protection from various environmental influences - drying, mechanical stress, heat loss and overheating.
phylogeny of the skeleton. Among invertebrates the outer skeleton is more common in the form of cuticular formations of the ectodermal epithelium. Such a skeleton is most developed in arthropods. It consists of chitin, protects the body from mechanical damage, drying out and serves as a site for muscle attachment.
At lower chordates(craniless) an internal axial skeleton appears in the form of a chord and dense fibrous strands supporting the fins and gill slits. The notochord is an elastic cord, consisting of special vacuolated cells (derivatives of the endoderm). It stretches along the dorsal side from the anterior end of the body to the posterior. An elastic sheath covers the chord surface. The supporting function of the chord is provided by the elasticity of the membranes and cell vacuoles, which maintain significant internal pressure (turgor) in the cells.
At higher chordates(vertebral) skeleton of a high degree of differentiation.
Axial skeleton. In lower vertebrates - cyclostomes and lower fish- notochord persists throughout life. But at the same time, the upper (in cyclostomes) and lower (in fish) arches of the vertebrae appear in the form of paired cartilages located metamerically above and below the notochord. Arcs have no functional value. At higher fish In addition to the arcs, the vertebral bodies develop - either due to the growth of the bases of the arcs, forming a ring of cartilaginous or bone tissue around the chord, or partly due to the arcs, and partly from the skeletal tissue surrounding the chord. After the formation of the vertebral body, arcs grow to it. The ends of the upper arches fuse between themselves, forming the canal of the spinal cord and the spinous process, the lower arches give lateral outgrowths (transverse processes). Thus, initially each vertebra consists of several elements. In fish, the chord is compressed by the vertebrae and takes the form of a beaded cord. The spine is differentiated into the trunk and tail sections. All vertebrae of the trunk region bear ribs. There are no ribs in the tail section.
in the spine amphibian two new departments are differentiated - cervical and sacral, each represented by one vertebra. The cervical region provides the mobility of the head, which is necessary in more difficult conditions of the terrestrial environment. The vertebra bears the ribs. The sacral region arises on the border of the caudal and trunk, gives support to the pelvic bones and hind limbs. The trunk section is represented by five vertebrae, which bear ribs of insignificant length. They do not reach the sternum and end freely.
At reptiles the number of sections of the spine increases; a new section appears - the lumbar. The number of vertebrae in the departments increases to 8-12. Progressive transformations take place in the cervical region. The body of the first cervical vertebra is not connected by arcs, but fuses with the body of the second cervical vertebra, forming an odontoid process. The first cervical vertebra takes the form of a ring and can rotate freely on the second vertebra, which dramatically increases the mobility of the head. The ribs in the cervical region are reduced. In the thoracic region, all vertebrae bear well-developed ribs. Most of them connect to the sternum to form the ribcage. The appearance of the chest provides a more perfect mechanism for breathing. The lumbar region is characterized by massive transverse processes formed by the growth of rudimentary ribs.
At mammals in the adult state, the notochord is preserved only in the form of the nucleus pulposus of the vertebrae. The spine consists of five sections - cervical, thoracic, lumbar, sacral, caudal. A constant number of vertebrae in the cervical region is characteristic, equal to ?. The ribs of the cervical vertebrae are completely reduced. In the thoracic region, the number of vertebrae ranges from 9 to 14, more often 12-13. The vertebrae bear ribs, most of them connected to the sternum. The lumbar region contains from 2 to 9 vertebrae with powerful transverse processes. The sacrum is formed by fused vertebrae, including 10 or more. The number of vertebrae in the caudal region varies.
Skeleton of the free limb. For the first time, limbs appear in fish in the form of paired fins - pectoral and ventral, which in the process of evolution are transformed into five-fingered limbs - the organs of movement of land animals.
In most fish, in the skeleton of the pectoral fin, a proximal section is distinguished, consisting of a small number (1-3) of relatively large cartilaginous plates, and a distal section, built from a large number of radially arranged thin rays. Each beam consists of a large number of small elements located along its axis. All parts of the fin skeleton are fixedly interconnected and form a single plane. The fin is fixedly connected to the shoulder girdle, since several elements of the proximal section are involved in the articulation. In the vast majority of fish, the fins cannot serve as a support for the body, but are used as a means to change the direction of movement (turns). The exception is the fins of fossils lobe-finned fish(Crossopterigia), widespread in the Devonian period (about 300 million years ago) and then extinct. Only one of the branches of the lobe-finned has survived to this day in the region of the southeast coast of Africa.
First amphibians(stegocephals) possessed five-fingered limbs. Their skeleton, according to the plan of structure and the ratio of bones, was very similar to the fins of the crossopterans (see Fig. 132, c). As in the lobe-finned fish, the proximal section is represented by one large element (shoulder), followed by 2 bone elements that make up the forearm, then 3-4 rows of small bones that maintain the correct radial arrangement (wrist). After the wrist follows the metacarpus (5 bones) and, finally, the phalanges of the fingers, which also retain the radial type of the location of the bones. Such a skeletal structure plan is the same for all terrestrial vertebrates.
Along with the simplification of the structure and a decrease in the number of elements, an important point in the process of transforming the fins into limbs of the terrestrial type was the replacement of a strong connection of the skeleton elements with each other by movable joints in the form of joints. As a result, the limb turned from a simple lever into a complex lever, the parts of which are movable relative to each other. The process of simplifying the skeleton of the lobe-finned limb continued later. The main changes affected the distal section. So there was a further decrease in the number of rays. The ancestors of terrestrial forms had 7 fingers connected by a membrane. When reaching land, the extreme fingers were reduced and turned into rudiments. The number of bone elements in the wrist also decreased. Amphibians have 3 rows of carpal bones - proximal, middle and distal. At higher vertebrates the middle row disappears, and the number of bones in each row decreases sequentially, as well as the phalanges. At the same time, in the process of evolution of terrestrial forms, a significant lengthening of the bones of the proximal sections - the shoulder, forearm, and also the distal section (fingers) occurs, while the bones of the middle section are shortened.
Human hand retains the plan of the structure of the limbs of the ancestors - the shoulder, forearm, wrist, metacarpus, phalanges of the fingers. At the same time, it has differences related to its new function - the transformation into a labor organ. Structural features and an exceptional variety of specific functions of the human hand arose in the process of mastering labor activity. The hand, therefore, as noted by F. Engels, is not only an organ, but also a product of labor.
Head skeleton(scull). The skull of vertebrates consists of 2 main sections - the axial and visceral skull. The axial section (cranial box) is a continuation of the axial skeleton and serves to protect the brain and sensory organs. The visceral region (facial skull) forms a support for the anterior part of the digestive tract.
Both parts of the skull develop independently of each other and in different ways. The most significant transformations in the process of evolution occur in the visceral skull, the elements of which are transformed into the jaw apparatus, and in the higher ones, in addition, they give rise to elements of the organ of hearing.
In the early stages of development, the visceral and axial sections of the skull are not connected, but later such a connection arises.
The anlages of the axial and visceral skull common to all embryos undergo changes in the process of postembryonic development in accordance with the peculiarities of the historical development of each class.
At lower fish The (cartilaginous) axial skull in adulthood encloses the brain more tightly. The occipital region appears, the auditory capsules are included in the side walls, the olfactory cartilages are attached to the front of the skull. The visceral skull consists of a number of cartilaginous visceral arches, covering the pharynx like a hoop (see Fig. 135), of which the 1st (maxillary) arch consists of only two large cartilages, elongated in the anterior-posterior direction - the upper (palatosquare) and lower (Meckel). The upper and lower cartilages of each side are fused together and perform the functions of the jaws (primary jaws). The 2nd visceral arch consists of two paired and one unpaired cartilage, connecting the paired cartilages from below to each other. The upper element of the pair, larger, is the hyomandibular cartilage, the lower paired element is the hyoid, and the unpaired element is the copula. The upper edge of the hyomandibular cartilage is connected to the cranium, the lower to the hyoid, and the anterior to the jaw arch lying in front. Thus, the hyomandibular cartilage acts as a suspension for the jaw arch, it is attached to the skull with the help of the hyoid arch. This type of connection of the jaws with the skull is called hyostyle (hyostyle skull) and is characteristic of lower vertebrates. The remaining arcs (3-7) form a support for the respiratory apparatus.
At higher fish(bone), along with the primary, cartilaginous skull, homologous to the axial skull of lower fish, a secondary skull of false bones appears. The secondary skull is much wider than the primary. It covers the primary skull from above (paired parietal, frontal, nasal bones), from below (large unpaired bone - parasphenoid) and from the sides (supratemporal, squamous bones). The main changes in the visceral skull concern the jaw arch. The upper jaw instead of one large palatine square cartilage consists of 5 elements - the palatine cartilage, the square bone and 3 pterygoid bones. In front of the primary upper jaw, 2 large false bones are formed - the premaxillary and maxillary, equipped with large teeth, which become the secondary upper jaws. The distal end of the primary mandible is also covered by a large dentary, which protrudes far anteriorly and forms the secondary mandible. Thus, the function of the jaws in higher fish passes to the secondary jaws formed by superimposed bones. The hyoid arch retains its former function of suspension of the jaws to the skull. Consequently, the skull of higher fishes is also hyostyle.
At amphibians significant changes relate mainly to the visceral region, since with the transition to a terrestrial lifestyle, gill respiration is replaced by skin-pulmonary respiration. The primary skull of amphibians hardly undergoes ossification and does not differ from the primary skull of fish. The secondary skull is characterized by a pronounced reduction in the number of bone elements.
With regard to the visceral skull, one of the main differences lies in the new way of connecting the jaw arch to the skull. Amphibians, unlike the hyostyle skull of fish, have an autostyle skull, that is, their jaw arch is connected to the skull
directly, without the help of the hyoid arch, due to the fusion of the palatine cartilage of the jaw arch (primary upper jaw) throughout with the axial skull. The mandibular region articulates with the maxillary and thus also receives a connection with the skull without the help of the hyoid arch. Thanks to this, the hyomandibular cartilage is released from the function of suspension of the jaws.
At reptile embryos four pairs of gill arches and gill slits are also laid, of which only one breaks out, namely the first, located between the jaw and hyoid arches, while the rest quickly disappear. The axial skull, unlike amphibians, consists only of bone tissue. The visceral skull of reptiles, like that of amphibians, is autostyle. However, there are also some differences. The anterior element of the primary upper jaw, the palatine cartilage, is reduced. Therefore, only the posterior part, the quadrate bone, is involved in the articulation of the upper jaw to the skull. Accordingly, the area of the attachment surface is reduced. The lower jaw is connected to the quadrate bone of the upper jaw and in this way is attached to the skull. The only gill gap that breaks out in the embryonic period is transformed into the middle ear cavity, and the hyomandibular cartilage into the auditory ossicle. The rest of the visceral skeleton forms the hyoid apparatus, which consists of the body of the hyoid bone and three pairs of processes. The body of the hyoid bone is formed by the fusion of the copulae of the hyoid arch and all gill arches. The anterior horns of this bone correspond to the lower paired element of the hyoid arch - the hyoid, and the posterior horns - to the paired elements of the first two gill arches.
In the axial skull of mammals, a decrease in the number of bones occurs due to their fusion. The configuration of the skull changes dramatically, which is associated with a progressive increase in brain volume. In particular, the anterior wall of the cranium approaches the olfactory capsules, the brain cavity gradually approaches the nasal cavity, and in forms with the most developed brain (humans) it turns out to be located above the nasal cavity, while in the lower forms the brain cavity is located behind the nasal cavity. The main feature of the visceral skull of mammals is the appearance of a fundamentally new type of articulation of the lower jaw with the skull, namely, the lower jaw is attached to the skull directly, forming a movable joint with the squamosal bone of the cranium. This articulation involves only the distal part of the integumentary dentary (secondary lower jaw). Its posterior end in mammals is curved upwards and ends with an articular process. Due to the formation of this joint, the square bone of the primary upper jaw loses its function as a suspension of the lower jaw and turns into an auditory bone, which is called the anvil (Fig. 137). The primary lower jaw in the process of embryonic development completely leaves the composition of the lower jaw and is also transformed into the auditory ossicle, which is called the malleus. And, finally, the upper part of the hyoid arch - the homologue of the hyomandibular cartilage - is transformed into the third auditory bone - the stirrup. Thus, in mammals, instead of one, three auditory ossicles are formed, which form a functionally single chain.
The lower part of the hyoid arch in mammals is transformed into the anterior horns of the hyoid bone. The first gill arch gives rise to the posterior horns, and its copula gives rise to the body of the hyoid bone; The 2nd and 3rd gill arches form the thyroid cartilage, which appears for the first time in the process of evolution in mammals, and the 4th and 5th gill arches provide material for the rest of the laryngeal cartilages, and also, possibly, for the tracheal ones.
As can be seen from the comparative anatomical review, human skeleton completely homologous to the mammalian skeleton. A person does not have a single bone that would be absent from representatives of the class (Fig. 138). At the same time, in the process of anthropogenesis, a number of features appear in the human skeleton. Most of them are directly or indirectly related to bipedalism. According to F. Engels, the transition to upright posture was the main factor that determined the restructuring of the human body.
A direct consequence of a person's transition to bipedal locomotion is:
1) changes in the foot, which lost its grasping function and turned into an organ with a purely supporting function, which was accompanied by the appearance of a longitudinal arch of the foot (absorbs concussion of internal organs when walking);
2) powerful development of the thumb (I) in comparison with others, since it becomes the main fulcrum, and the loss of significant mobility and ability to oppose them;
3) S-shaped bend of the spine, softening the shocks of the internal organs when walking;
4) tilt of the pelvis at an angle of 60 ° to the horizontal due to the movement of the center of gravity;
5) movement of the foramen magnum and a change in the position of the head relative to the spine;
6) the appearance of the mastoid process of the temporal bone - the place of attachment of the sternocleidomastoid muscle, which holds the head in a vertical position.
Indirectly connected with bipedalism are: specialization of the upper limbs as an organ of labor in connection with their release from the function of movement; features of the brain skull; characteristic body proportions are shorter arms and longer legs.
Regardless of the changes associated with upright posture, there was a formation of the chin protrusion of the lower jaw, which arose in connection with articulate speech.
The process of adapting a person to upright posture has not yet ended, as evidenced by the relatively frequent cases of hernia when lifting heavy weights, prolapse of the uterus.
104. Comparative review of the skeleton of vertebrates. Head skeleton. Axial skeleton. limb skeleton. The main trends of progressive evolution. Congenital malformations of the human skeleton.
The phylogenesis of motor function underlies the progressive evolution of animals. Therefore, the level of their organization primarily depends on the nature of motor activity, which is determined by the characteristics of the organization. musculoskeletal system, undergone major evolutionary transformations in the Chordata type due to a change in habitats and changes in the forms of locomotion. Indeed, the aquatic environment in animals that do not have an external skeleton suggests uniform movements due to the bends of the whole body, while life on land is more conducive to their movement with the help of limbs.
Consider separately the evolution of the skeleton and the muscular system.
14.2.1. Skeleton
In chordates internal skeleton. According to the structure and functions, it is divided into axial, skeleton of the limbs and head.
14.2.1.1. Axial skeleton
In the subtype Cranial there is only axial skeleton in the form of a chord. It is built from highly vacuolated cells, tightly adjacent to each other and covered on the outside by common elastic and fibrous membranes. The elasticity of the chord is given by the turgor pressure of its cells and the strength of the membranes. The notochord is laid down in the ontogeny of all chordates and, in more highly organized animals, performs not so much a support function as a morphogenetic one, being an organ that carries out embryonic induction.
Throughout life in vertebrates, the notochord is preserved only in cyclostomes and some lower fish. In all other animals, it is reduced. In humans, in the postembryonic period, the rudiments of the notochord are preserved in the form of the nucleuspulposus of the intervertebral discs. Preservation of an excess amount of chordal material in case of violation of its reduction is fraught with the possibility of developing tumors in humans - chord, arising from it.
In all vertebrates, the notochord is gradually replaced vertebrae developing from somite sclerotomes, and is functionally replaced spinal column. This is one of the most pronounced examples of homotopic substitution of organs (see § 13.4). The formation of vertebrae in phylogeny begins with the development of their arcs, covering the neural tube and becoming places of muscle attachment. Starting with cartilaginous fish, cartilage of the notochord membrane and growth of the bases of the vertebral arches are found, as a result of which the vertebral bodies are formed. The fusion of the upper vertebral arches above the neural tube forms the spinous processes and the spinal canal, which contains the neural tube (Fig. 14.6).
Rice. 14.6. Development of the vertebra. A-early stage; B- subsequent stage:
1 -chord, 2- chord shell, 3- upper and lower vertebral arches, 4- spinous process, 5- ossification zones, 6-rudiment of chord, 7 - cartilaginous body of the vertebra
Replacement of the chord with the vertebral column - a more powerful supporting organ with a segmental structure - allows you to increase the overall size of the body and activates the motor function. Further progressive changes in the spinal column are associated with tissue substitution - the replacement of cartilage tissue with bone tissue, which is found in bony fish, as well as with its differentiation into sections.
Fish have only two sections of the spine: trunk and tail. This is due to their movement in the water due to the bends of the body.
Amphibians also acquire cervical and sacral departments, each represented by one vertebra. The first provides greater head mobility, and the second provides support for the hind limbs.
In reptiles, the cervical spine is elongated, the first two vertebrae of which are movably connected to the skull and provide greater head mobility. Appears lumbar department, still weakly delimited from the thoracic, and the sacrum already consists of two vertebrae.
Mammals are characterized by a stable number of vertebrae in the cervical region, equal to 7. Due to the great importance in the movement of the hind limbs, the sacrum is formed by 5-10 vertebrae. The lumbar and thoracic regions are clearly separated from each other.
In fish, all trunk vertebrae bear ribs that do not fuse with each other and with the sternum. They give the body a stable shape and provide support for the muscles that bend the body in a horizontal plane. This function of the ribs is preserved in all vertebrates that perform serpentine movements - in caudate amphibians and reptiles, therefore their ribs are also located on all vertebrae, except for the caudal ones.
In reptiles, part of the ribs of the thoracic region fuses with the sternum, forming the chest, and in mammals, the chest contains 12-13 pairs of ribs.
Rice. 14.7. Anomalies in the development of the axial skeleton. A - rudimentary cervical ribs (shown by arrows); B - nonunion of the spinous processes of the vertebrae in the thoracic and lumbar regions. Spinal hernias
The ontogenesis of the human axial skeleton recapitulates the main phylogenetic stages of its formation: in the period of neurulation, a notochord is formed, which is subsequently replaced by a cartilaginous and then a bone spine. A pair of ribs develops on the cervical, thoracic and lumbar vertebrae, after which the cervical and lumbar ribs are reduced, and the thoracic ribs fuse in front with each other and with the sternum, forming the chest.
Violation of the ontogenesis of the axial skeleton in humans can be expressed in such atavistic malformations as nonunion of the spinous processes of the vertebrae, resulting in the formation of spinabifida- spinal defect. In this case, the meninges often protrude through the defect and form spinal hernia(Fig. 14.7).
At the age of 1.5-3 months. the human embryo has a caudal spine, consisting of 8-11 vertebrae. Violation of their reduction subsequently explains the possibility of such a well-known anomaly of the axial skeleton as tail persistence.
Violation of the reduction of the cervical and lumbar ribs underlies their preservation in postnatal ontogenesis.
