The musculoskeletal system ensures the movement and preservation of the position of the animal's body in space, forms the external shape of the body and participates in metabolic processes. It accounts for about 60% of the body weight of an adult animal.

Conditionally, the musculoskeletal system is divided into passive and active parts. To passive part include bones and their joints, on which the nature of the mobility of bone levers and links of the animal's body depends (15%). active part make up skeletal muscles and their auxiliary attachments, due to the contractions of which, the bones of the skeleton are set in motion (45%). Both the active and passive parts have a common origin (mesoderm) and are closely related.

Functions of the apparatus of movement:

1) Motor activity is a manifestation of the vital activity of the organism, it is it that distinguishes animal organisms from plant organisms and causes the emergence of a wide variety of modes of movement (walking, running, climbing, swimming, flying).

2) The musculoskeletal system forms the shape of the body - exterior animal, since its formation took place under the influence of the gravitational field of the Earth, then its size and shape in vertebrates differ in significant diversity, which is explained by different conditions of their habitat (terrestrial, terrestrial-tree, air, water).

3) In addition, the apparatus of movement provides a number of vital functions of the body: the search for and capture of food; attack and active defense; carries out the respiratory function of the lungs (respiratory motility); helps the heart with the promotion of blood and lymph in the vessels ("peripheral heart").

4) In warm-blooded animals (birds and mammals), the apparatus of movement ensures the preservation of a constant body temperature;

The functions of the apparatus of movement are provided by the nervous and cardiovascular systems., respiratory organs, digestion and urination, skin, endocrine glands. Since the development of the apparatus of movement is inextricably linked with the development of the nervous system, when these connections are violated, first paresis and then paralysis movement apparatus (the animal cannot move). With a decrease physical activity there is a violation of metabolic processes and atrophy of muscle and bone tissues.

The organs of the musculoskeletal system have properties of elastic deformations, when moving, mechanical energy arises in them in the form of elastic deformations, without which normal blood circulation and impulses of the brain and spinal cord cannot be carried out. The energy of elastic deformations in the bones is converted into piezoelectric, and in the muscles - into heat. The energy released during movement displaces blood from the vessels and causes irritation of the receptor apparatus, from which nerve impulses enter the central nervous system. Thus, the work of the movement apparatus is closely connected and cannot be carried out without the nervous system, and the vascular system, in turn, cannot function normally without the movement apparatus.

The basis of the passive part of the apparatus of movement is the skeleton. Skeleton (Greek sceletos - dried up, dried; Lat. Skeleton) are bones connected in a certain order that form a solid frame (skeleton) of the animal's body. Since in Greek the bone is "os", the science of the skeleton is called osteology.

The skeleton consists of about 200-300 bones (Horse, k.s. -207-214; pig, dog, cat -271-288), which are interconnected by means of connective, cartilaginous or bone tissue. The mass of the skeleton in an adult animal is from 6% (pig) to 15% (horse, k.r.s.).

All skeletal functions can be divided into two large groups: mechanical and biological. To mechanical functions include: protective, support, locomotor, spring, anti-gravity, and biological - metabolism and hematopoiesis (hemocytopoiesis).

1) The protective function is that the skeleton forms the walls of the body cavities in which the vital organs are located. So, for example, in the cranial cavity is the brain, in the chest - the heart and lungs, in the pelvic cavity - the genitourinary organs.

2) The supporting function lies in the fact that the skeleton is a support for the muscles and internal organs, which, being attached to the bones, are held in their position.

3) The locomotor function of the skeleton is manifested in the fact that the bones are levers that are set in motion by the muscles and ensure the movement of the animal.

4) The spring function is due to the presence in the skeleton of formations that soften shocks and tremors (cartilaginous pads, etc.).

5) The anti-gravitational function is manifested in the fact that the skeleton creates a support for the stability of the body rising above the ground.

6) Participation in metabolism, especially in mineral metabolism, since bones are a depot of mineral salts of phosphorus, calcium, magnesium, sodium, barium, iron, copper and other elements.

7) Buffer function. The skeleton acts as a buffer that stabilizes and maintains a constant ionic composition of the body's internal environment (homeostasis).

8) Participation in hemocytopoiesis. Located in the bone marrow cavities, red bone marrow produces blood cells. The mass of bone marrow in relation to the mass of bones in adult animals is approximately 40-45%.

DIVISION OF THE SKELETON

The skeleton is the frame of the animal's body. It is usually divided into main and peripheral.

to the axial skeleton include the skeleton of the head (cranial cranium), the skeleton of the neck, trunk and tail. The skull has the most complex structure, since it contains the brain, organs of vision, smell, balance and hearing, oral and nasal cavity. The main part of the skeleton of the neck, trunk and tail is the spinal column (columna vertebralis).

The spinal column is divided into 5 sections: cervical, thoracic, lumbar, sacral and caudal. The cervical region consists of the cervical vertebrae (v.cervicalis); the thoracic region - from the thoracic vertebrae (v.thoracica), ribs (costa) and sternum (sternum); lumbar - from the lumbar vertebrae (v.lumbalis); sacral - from the sacrum (os sacrum); tail - from the tail vertebrae (v.caudalis). The thoracic region of the body has the most complete structure, where there are thoracic vertebrae, ribs, breast bone, which together form the chest (thorax), in which the heart, lungs, and mediastinal organs are located. The smallest development, in terrestrial animals, is the tail section, which is associated with the loss of the locomotor function of the tail during the transition of animals to a terrestrial lifestyle.

The axial skeleton is subject to the following patterns of body structure, which ensure the mobility of the animal. They include :

1) Bipolarity (uniaxiality) is expressed in the fact that all sections of the axial skeleton are located on the same axis of the body, moreover, the skull is on the cranial pole, and the tail is on the opposite. The sign of uniaxiality makes it possible to establish two directions in the animal's body: cranial - towards the head and caudal - towards the tail.

2) Bilaterality (bilateral symmetry) is characterized by the fact that the skeleton, as well as the trunk, can be divided by the sagittal, medial plane into two symmetrical halves (right and left), in accordance with this, the vertebrae will be divided into two symmetrical halves. Bilaterality (antimeria) makes it possible to distinguish lateral (lateral, external) and medial (internal) directions on the body of an animal.

3) Segmentation (metamerism) is that the body can be divided by segmental planes into a certain number of relatively identical metamers - segments. Metameres follow the axis from front to back. On the skeleton, such metameres are vertebrae with ribs.

4) Tetrapodia is the presence of 4 limbs (2 thoracic and 2 pelvic)

5) And the last pattern is, due to gravity, the location in the spinal canal of the neural tube, and under it the intestinal tube with all its derivatives. In this regard, a dorsal direction is planned on the body - towards the back and a ventral direction - towards the abdomen.

peripheral skeleton represented by two pairs of limbs: thoracic and pelvic. In the skeleton of the limbs, there is only one regularity - bilaterality (antimerism). The limbs are paired, there are left and right limbs. The rest of the elements are asymmetrical. On the limbs, belts (thoracic and pelvic) and the skeleton of free limbs are distinguished.

With the help of a belt, the free limb is attached to the spinal column. Initially, the limb girdle had three pairs of bones: the scapula, the clavicle and the coracoid bone (everything was preserved in birds), only one scapula remained in animals, only a process on the tubercle of the scapula from the medial side was preserved from the coracoid bone, rudiments of the clavicle are present in predators (dog and cat). In the pelvic girdle, all three bones (iliac, pubic and ischial) are well developed, which grow together.

The skeleton of free limbs has three links. The first link (stilopodium) has one beam (Greek stilos - column, podos - leg): on the thoracic limb - this is the humerus, on the pelvic - the femur. The second links (zeugopodium) are represented by two rays (zeugos - a pair): on the thoracic limb - these are the radius and ulna (bones of the forearm), on the pelvic - the tibia and fibula (bones of the lower leg). The third links (autipodium) form: on the thoracic limb - the hand, on the pelvic - the foot. They distinguish between the basipodium (the upper section is the bones of the wrist and, accordingly, the tarsus), the metapodium (the middle section is the bones of the metacarpus and metatarsus) and the acropodium (the most extreme section is the phalanges of the fingers).

PHYLOGENESIS OF THE SKELETON

In the phylogeny of vertebrates, the skeleton develops in two directions: external and internal.

The external skeleton performs a protective function, is characteristic of lower vertebrates and is located on the body in the form of scales or shells (tortoise, armadillo). In higher vertebrates, the external skeleton disappears, but some of its elements remain, changing their purpose and location, becoming the integumentary bones of the skull and, located already under the skin, are associated with the internal skeleton. In phylo-ontogenesis, such bones go through only two stages of development (connective tissue and bone) and are called primary. They are not able to regenerate - if the bones of the skull are injured, they are forced to be replaced with artificial plates.

The internal skeleton performs mainly a supporting function. In the course of development under the influence of biomechanical load, it constantly changes. If we consider invertebrates, then their internal skeleton looks like partitions to which muscles are attached.

The primitive chordates animals (lancelet ), along with partitions, an axis appears - a chord (cell strand), dressed in connective tissue membranes.

At cartilaginous fish(sharks, rays), cartilaginous arches are segmentally formed around the notochord, which later form the vertebrae. Cartilaginous vertebrae, connecting with each other, form the spinal column, ventrally, ribs join it. Thus, the notochord remains in the form of nucleus pulposus between the vertebral bodies. At the cranial end of the body, a skull is formed and, together with the spinal column, participates in the formation of the axial skeleton. In the future, the cartilaginous skeleton is replaced by a bone, less flexible, but more durable.

At bony fish the axial skeleton is built from more durable - coarse fibrous bone tissue, which is characterized by the presence of mineral salts and a disorderly arrangement of collagen (ossein) fibers in the amorphous component.

With the transition of animals to a terrestrial way of life, amphibian a new part of the skeleton is formed - the skeleton of the limbs. As a result of this, in terrestrial animals, in addition to the axial skeleton, the peripheral skeleton (the skeleton of the limbs) is also formed. In amphibians, as in bony fish, the skeleton is built of coarse fibrous bone tissue, but in more highly organized terrestrial animals (reptiles, birds and mammals) the skeleton is already built from lamellar bone tissue, consisting of bone plates containing collagen (ossein) fibers arranged in an orderly manner.

Thus, the internal skeleton of vertebrates passes through three stages of development in phylogenesis: connective tissue (membranous), cartilaginous and bone. The bones of the internal skeleton that go through all these three stages are called secondary (primordial).

ONTOGENESIS OF THE SKELETON

In accordance with the basic biogenetic law of Baer and E. Haeckel, the skeleton also goes through three stages of development in ontogenesis: membranous (connective tissue), cartilaginous and bone.

At the earliest stage of development of the embryo, the supporting part of its body is dense connective tissue, which forms a membranous skeleton. Then a chord appears in the embryo, and around it the cartilaginous, and later the bony vertebral column and skull, and then the limbs begin to form.

In the prefetal period, the entire skeleton, with the exception of the primary integumentary bones of the skull, is cartilaginous and makes up about 50% of body weight. Each cartilage has the shape of a future bone and is covered with a perichondrium (dense connective tissue sheath). During this period, ossification of the skeleton begins, i.e. formation of bone tissue in place of cartilage. Ossification or ossification (Latin os - bone, facio - I do) occurs both from the outer surface (perichondral ossification) and from the inside (endochondral ossification). In place of the cartilage, coarse-fibrous bone tissue is formed. As a result of this, the skeleton of the fetus is built of coarse fibrous bone tissue.

Only in the neonatal period, coarse fibrous bone tissue is replaced by a more perfect lamellar bone tissue. During this period, special attention is required for newborns, since their skeleton is not yet strong. As for the chord, its remains are located in the center of the intervertebral discs in the form of pulpous nuclei. Particular attention during this period should be paid to the integumentary bones of the skull (occipital, parietal and temporal), as they bypass the cartilaginous stage. Significant connective tissue spaces, called fontanelles (fonticulus), are formed between them in ontogenesis, only in old age they are completely ossified (endesmal ossification).



The concept of " phylogenesis”(from the Greek phyle - “genus, tribe” and genesis - “birth, origin”) was introduced in 1866 by the German biologist Ernst Haeckel to denote the historical development of organisms in the process of evolution.

Consider how the spine developed and improved from the simplest organisms to humans. It is necessary to distinguish between the external and internal skeleton.

Exterior skeleton performs a protective function. It is inherent in lower vertebrates and is located on the body in the form of scales or shells (tortoise, armadillo). In higher vertebrates, the external skeleton disappears, but its individual elements remain, changing their purpose and location, becoming the integumentary bones of the skull. Located already under the skin, they are connected with the internal skeleton.

Internal skeleton performs mainly a supporting function. In the course of development, under the influence of a biomechanical load, it constantly changes. In invertebrates, it looks like partitions to which muscles are attached.

In primitive chordates (lancelets), along with partitions, an axis appears - a chord (cell cord), dressed in connective tissue membranes. In fish, the spine is relatively simple and consists of two sections (trunk and tail). Their soft cartilaginous spine is more functional than that of chordates; the spinal cord is located in the vertebral canal. The skeleton of fish is more perfect, allowing for faster and more precise movements with a smaller mass.

With the transition to a terrestrial way of life, a new part of the skeleton is formed - the skeleton of the limbs. And if in amphibians the skeleton is made of coarse fibrous bone tissue, then in more highly organized terrestrial animals it is already built from lamellar bone tissue, consisting of bone plates containing ordered collagen fibers.

The internal skeleton of vertebrates passes through three stages of development in phylogenesis: connective tissue (membranous), cartilaginous and bone.

Mammal skeleton (left) and fish (right)

The deciphering of the lancelet genome, completed in 2008, confirmed the proximity of the lancelets to the common ancestor of vertebrates. According to the latest scientific data, lancelets are relatives of vertebrates, although the most distant.

The mammalian spine consists of the cervical, thoracic, lumbar, sacral, and caudal sections. Its characteristic feature is the platycelial (having flat surfaces) shape of the vertebrae, between which cartilaginous intervertebral discs are located. The upper arches are well defined.

In the cervical region, all mammals have 7 vertebrae, the length of which depends on the length of the neck. The only exceptions are two animals: the manatee has 6 of these vertebrae, and in different species of sloths - from 8 to 10. The giraffe has very long cervical vertebrae, while cetaceans that do not have a cervical interception, on the contrary, are extremely short.

The ribs are attached to the vertebrae of the thoracic region, forming the chest. The sternum closing it is flat and only in bats and in representatives of burrowing species with powerful forelimbs (for example, moles) has a small crest (keel), to which the pectoral muscles are attached. In the thoracic region there are 9-24 (usually 12-15) vertebrae, the last 2-5 bear false ribs that do not reach the sternum.

In the lumbar region from 2 to 9 vertebrae; rudimentary ribs merge with their large transverse processes. The sacral region is formed by 4-10 fused vertebrae, of which only the first two are truly sacral, and the rest are caudal. The number of free tail vertebrae ranges from 3 (in the gibbon) to 49 (in the long-tailed pangolin).

The mobility of individual vertebrae depends on lifestyle. So, in small running and climbing animals, it is high along the entire length of the spine, so their body can bend in different directions and even curl up into a ball. The thoracic and lumbar vertebrae are less mobile in large, rapidly moving animals. In mammals that move hind legs(kangaroos, jerboas, jumpers), the largest vertebrae are located at the base of the tail and sacrum, and then their size consistently decreases. In ungulates, on the contrary, the vertebrae and especially their spinous processes are larger in the anterior part of the thoracic region, where the powerful muscles of the neck and partly of the forelimbs are attached to them.

In birds, the forelimbs (wings) are adapted for flying, and the hind limbs for moving on the ground. A peculiar feature of the skeleton is the pneumaticity of the bones: they are lighter because they contain air. The bones of birds are also quite fragile, as they are rich in lime salts, and therefore the strength of the skeleton is largely achieved by the fusion of many bones.

The musculoskeletal system ensures the movement and preservation of the position of the animal's body in space, forms the external shape of the body and participates in metabolic processes. It accounts for about 60% of the body weight of an adult animal.
Conditionally, the musculoskeletal system is divided into passive and active parts. The passive part includes bones and their joints, on which the nature of the mobility of the bone levers and links of the animal's body depends (15%). The active part is made up of skeletal muscles and their accessories, due to the contractions of which, the bones of the skeleton are set in motion (45%). Both the active and passive parts have a common origin (mesoderm) and are closely related.

Movement apparatus functions:

1) Motor activity is a manifestation of the vital activity of the organism, it is it that distinguishes animal organisms from plant organisms and causes the emergence of a wide variety of modes of movement (walking, running, climbing, swimming, flying).
2) The musculoskeletal system forms the shape of the body - the exterior of the animal, since its formation took place under the influence of the gravitational field of the Earth, then its size and shape in vertebrates differ in significant diversity, which is explained by different conditions of their habitat (terrestrial, terrestrial-arboreal, air , water).
3) In addition, the apparatus of movement provides a number of vital functions of the body: the search for and capture of food; attack and active defense; carries out the respiratory function of the lungs (respiratory motility); helps the heart with the promotion of blood and lymph in the vessels ("peripheral heart").
4) In warm-blooded animals (birds and mammals), the apparatus of movement ensures the preservation of a constant body temperature;
The functions of the apparatus of movement are provided by the nervous and cardiovascular systems, respiratory, digestive and urinating organs, skin, endocrine glands. Since the development of the apparatus of movement is inextricably linked with the development of the nervous system, if these connections are violated, first paresis occurs, and then paralysis of the apparatus of movement (the animal cannot move). With a decrease in physical activity, there is a violation of metabolic processes and atrophy of muscle and bone tissues.
The organs of the musculoskeletal system have the properties of elastic deformations; when moving, mechanical energy arises in them in the form of elastic deformations, without which normal blood circulation and impulses of the brain and spinal cord cannot be carried out. The energy of elastic deformations in the bones is converted into piezoelectric, and in the muscles - into heat. The energy released during movement displaces blood from the vessels and causes irritation of the receptor apparatus, from which nerve impulses enter the central nervous system. Thus, the work of the movement apparatus is closely connected and cannot be carried out without the nervous system, and the vascular system, in turn, cannot function normally without the movement apparatus.

Skeleton

The basis of the passive part of the apparatus of movement is the skeleton. Skeleton (Greek sceletos - dried up, dried; Lat. Skeleton) are bones connected in a certain order that form a solid frame (skeleton) of the animal's body. Since the Greek word for bone is os, the science of the skeleton is called osteology.
The skeleton consists of about 200-300 bones (Horse -207), which are interconnected by means of connective, cartilaginous or bone tissue. The mass of the skeleton in an adult animal is 15%.
All functions of the skeleton can be divided into two large groups: mechanical and biological. The mechanical functions include: protective, supporting, locomotor, spring, anti-gravity, and the biological functions include metabolism and hematopoiesis (hemocytopoiesis).
1) The protective function is that the skeleton forms the walls of the body cavities in which the vital organs are located. So, for example, in the cranial cavity is the brain, in the chest - the heart and lungs, in the pelvic cavity - the genitourinary organs.
2) The supporting function lies in the fact that the skeleton is a support for the muscles and internal organs, which, being attached to the bones, are held in their position.
3) The locomotor function of the skeleton is manifested in the fact that the bones are levers that are set in motion by the muscles and ensure the movement of the animal.
4) The spring function is due to the presence in the skeleton of formations that soften shocks and tremors (cartilaginous pads, etc.).
5) The anti-gravitational function is manifested in the fact that the skeleton creates a support for the stability of the body rising above the ground.
6) Participation in metabolism, especially in mineral metabolism, since bones are a depot of mineral salts of phosphorus, calcium, magnesium, sodium, barium, iron, copper and other elements.
7) Buffer function. The skeleton acts as a buffer that stabilizes and maintains a constant ionic composition of the body's internal environment (homeostasis).
8) Participation in hemocytopoiesis. Located in the bone marrow cavities, red bone marrow produces blood cells. The mass of bone marrow in relation to the mass of bones in adult animals is approximately 40-45%.

The spinal column is divided into 5 sections: cervical, thoracic, lumbar, sacral and caudal. The cervical region consists of the cervical vertebrae (v.cervicalis); the thoracic region - from the thoracic vertebrae (v.thoracica), ribs (costa) and sternum (sternum); lumbar - from the lumbar vertebrae (v.lumbalis); sacral - from the sacrum (os sacrum); tail - from the tail vertebrae (v.caudalis). The thoracic region of the body has the most complete structure, where there are thoracic vertebrae, ribs, breast bone, which together form the chest (thorax), in which the heart, lungs, and mediastinal organs are located. The smallest development, in terrestrial animals, is the tail section, which is associated with the loss of the locomotor function of the tail during the transition of animals to a terrestrial lifestyle.
The axial skeleton is subject to the following patterns of body structure, which ensure the mobility of the animal. These include:
1) Bipolarity (uniaxiality) is expressed in the fact that all sections of the axial skeleton are located on the same axis of the body, moreover, the skull is on the cranial pole, and the tail is on the opposite. The sign of uniaxiality makes it possible to establish two directions in the animal's body: cranial - towards the head and caudal - towards the tail.
2) Bilaterality (bilateral symmetry) is characterized by the fact that the skeleton, as well as the trunk, can be divided by the sagittal, medial plane into two symmetrical halves (right and left), in accordance with this, the vertebrae will be divided into two symmetrical halves. Bilaterality (antimeria) makes it possible to distinguish lateral (lateral, external) and medial (internal) directions on the body of an animal.
3) Segmentation (metamerism) is that the body can be divided by segmental planes into a certain number of relatively identical metamers - segments. Metameres follow the axis from front to back. On the skeleton, such metameres are vertebrae with ribs.
4) Tetrapodia is the presence of 4 limbs (2 thoracic and 2 pelvic)
5) And the last pattern is, due to gravity, the location in the spinal canal of the neural tube, and under it the intestinal tube with all its derivatives. In this regard, a dorsal direction is planned on the body - towards the back and a ventral direction - towards the abdomen.

The peripheral skeleton is represented by two pairs of limbs: thoracic and pelvic. In the skeleton of the limbs, there is only one regularity - bilaterality (antimerism). The limbs are paired, there are left and right limbs. The rest of the elements are asymmetrical. On the limbs, belts (thoracic and pelvic) and the skeleton of free limbs are distinguished.

Phylogeny of the skeleton

In the phylogeny of vertebrates, the skeleton develops in two directions: external and internal.
The external skeleton performs a protective function, is characteristic of lower vertebrates and is located on the body in the form of scales or shells (tortoise, armadillo). In higher vertebrates, the external skeleton disappears, but some of its elements remain, changing their purpose and location, becoming the integumentary bones of the skull and, located already under the skin, are associated with the internal skeleton. In phylo-ontogenesis, such bones go through only two stages of development (connective tissue and bone) and are called primary. They are not able to regenerate - if the bones of the skull are injured, they are forced to be replaced with artificial plates.
The internal skeleton performs mainly a supporting function. In the course of development under the influence of biomechanical load, it constantly changes. If we consider invertebrates, then their internal skeleton looks like partitions to which muscles are attached.
In primitive chordate animals (lancelet), along with partitions, an axis appears - a chord (cellular strand), dressed in connective tissue membranes.
In cartilaginous fish (sharks, rays), cartilaginous arches are already segmentally formed around the notochord, which later form vertebrae. Cartilaginous vertebrae, connecting with each other, form the spinal column, ventrally, ribs join it. Thus, the notochord remains in the form of nucleus pulposus between the vertebral bodies. At the cranial end of the body, a skull is formed and, together with the spinal column, participates in the formation of the axial skeleton. In the future, the cartilaginous skeleton is replaced by a bone, less flexible, but more durable.
In bony fish, the axial skeleton is built from a stronger, coarse-fibrous bone tissue, which is characterized by the presence of mineral salts and a disorderly arrangement of collagen (ossein) fibers in the amorphous component.
With the transition of animals to a terrestrial way of life, a new part of the skeleton is formed in amphibians - the skeleton of the limbs. As a result of this, in terrestrial animals, in addition to the axial skeleton, the peripheral skeleton (the skeleton of the limbs) is also formed. In amphibians, as well as in bony fish, the skeleton is built from coarse fibrous bone tissue, but in more highly organized terrestrial animals (reptiles, birds and mammals) the skeleton is already built from lamellar bone tissue, consisting of bone plates containing collagen (ossein) fibers arranged in an orderly manner.
Thus, the internal skeleton of vertebrates passes through three stages of development in phylogenesis: connective tissue (membranous), cartilaginous and bone. The bones of the internal skeleton that go through all these three stages are called secondary (primordial).

Ontogeny of the skeleton

In accordance with the basic biogenetic law of Baer and E. Haeckel, the skeleton also goes through three stages of development in ontogenesis: membranous (connective tissue), cartilaginous and bone.
At the earliest stage of development of the embryo, the supporting part of its body is dense connective tissue, which forms a membranous skeleton. Then a chord appears in the embryo, and around it the cartilaginous, and later the bony vertebral column and skull, and then the limbs begin to form.
In the prefetal period, the entire skeleton, with the exception of the primary integumentary bones of the skull, is cartilaginous and makes up about 50% of body weight. Each cartilage has the shape of a future bone and is covered with a perichondrium (dense connective tissue sheath). During this period, ossification of the skeleton begins, i.e. formation of bone tissue in place of cartilage. Ossification or ossification (Latin os-bone, facio-do) occurs both from the outer surface (perichondral ossification) and from the inside (endochondral ossification). In place of the cartilage, coarse-fibrous bone tissue is formed. As a result of this, the skeleton of the fetus is built of coarse fibrous bone tissue.
Only in the neonatal period, coarse fibrous bone tissue is replaced by a more perfect lamellar bone tissue. During this period, special attention is required for newborns, since their skeleton is not yet strong. As for the chord, its remains are located in the center of the intervertebral discs in the form of pulpous nuclei. Particular attention during this period should be paid to the integumentary bones of the skull (occipital, parietal and temporal), as they bypass the cartilaginous stage. Significant connective tissue spaces, called fontanelles (fonticulus), are formed between them in ontogenesis, only in old age they are completely ossified (endesmal ossification).

Vertebral column: structure, development, specific features

In its development, the spinal column (columna vertebralis) is formed around the spinal cord, forming a bone container for it. In addition to protecting the spinal cord, the spinal column performs other important functions in the body: it is a support for the organs and tissues of the body, supports the head, participates in the formation of the walls of the chest, abdominal cavity and pelvis.

vertebral column(columna vertebralis) consists of separate elements - vertebrae (vertebra). Each vertebra has: a body (corpus vertebrae), a head (caput vertebrae), a fossa (fossa vertebrae), a ventral crest (crista ventralis), an arch (arcus vertebrae), and a vertebral opening (foramen vertebrae) is formed between the arch and the body. All the openings of the vertebrae together form the spinal canal (canalis vertebralis) for the spinal cord, and the caudal and cranial vertebral notches (incisures caudalis et cranialis) form the intervertebral foramen (foramen intervertebrale) for the nerves and blood vessels. The cranial and caudal articular processes (processus articularis cranialis et caudalis) protrude along the edges of the arches, which serve to articulate the vertebrae with each other. The spinous process (processus spinosus) protrudes - fixing muscles and ligaments.

The vertebral column is divided into cervical, thoracic, lumbar, sacral and caudal regions. The transverse processes (processus transversus) in the thoracic region are needed for the articulation of the vertebrae with the ribs, and the transverse costal, mastoid and spinous processes (processus costo-transversarium, mamillaris, spinosus) are needed for muscle attachment.

The number of vertebrae in each department is different and depends on the species characteristics of the animals. So, in the cervical region, most mammals (except for the sloth and manatee) have 7 vertebrae. They are divided into: 1st - atlas, 2nd - epistrophy, 3rd, 4th, 5th - typical, 6th, 7th.

· 1st(atlas - atlas), consists of two arches (arcus dorsalis et ventralis), on them, respectively, tubercles (tuberculum dorsale et ventrale). The transverse processes form the wings of the atlas (ala atlantis). Under the wing there is an atlas fossa (fossa atlantis), on the wings there are two pairs of holes for vessels and nerves - the wing (foramen alare) and the intervertebral (foramen intervertebrale), there are cranial and caudal articular fossae (fovea articularis cranialis et caudalis). FEATURES: there are no transverse holes on the atlas of the domestic bull.

· 2nd(axial epistrophy - axis), characterized by the presence of a tooth (dens) instead of the head of the vertebra and a ridge (crista dorsalis) instead of the spinous process, also the transverse process (processus transversus) is single.

· 3rd, 4th, 5th- typical. - their transverse processes fused with the costal, forming - transverse costal (processus costo-transversarium), and the spinous processes are tilted towards the head.

· 6th and 7th vertebrae - differ from the rest in shape and are atypical. 6th - instead of a ventral crest, it has a massive ventral plate (lamina ventralis). 7th - does not have a transverse opening, but has caudal costal fossae (fovea costalis caudalis) on the vertebral body.

In the thoracic spine of cattle and dogs, 13 vertebrae each, in pigs 14-17, in horses 18. The thoracic vertebrae (vertebrae thoracicae), together with the ribs and sternum, form the chest. The vertebrae of this department have caudal and cranial costal fossae (fovea costalis caudalis et cranialis), costal facets on the transverse processes (fovea costalis processus transversalis). The spinous process (processus spinosus) is tilted back towards the tail. The spinous processes of the 2nd to 9th vertebrae form the basis of the withers (regio interscapularis). The spinous process of the 13th (12th in a pig, 16th in a horse, and 11th in a dog) vertebra stands vertically - diaphragmatic. On the transverse processes (processus transversus) are mastoid processes (processus mamillaris).

AT lumbar of the spine in cattle and horses, 6 vertebrae each, in pigs and dogs, 7 each. Lumbar vertebrae (vertebrae lumbales), characterized by the presence of long, flat transverse processes and well-developed articular processes. transverse processes with sharp, uneven edges and bent forward towards the head. The spinous processes stand vertically. The cranial articular processes form semi-cylindrical bushings, and the caudal processes form the same blocks.

AT sacral region The vertebrae of the spine (vertebrae sacrales) fuse into one bone - the sacrum (os sacrum), which consists of 5 vertebrae in cattle and horses, 4 in pigs, and 3 in dogs.

The spinous processes have merged into the medial sacral crest (crista sacralis mediana), there are no interannual foramen. Intervertebral notches formed 4 pairs of dorsal and ventral sacral foramina (foramina sacralia dorsalia et ventralia). The transverse processes have merged - jagged lateral parts (partes lateralis). The first two transverse processes formed the wings of the sacrum (ala sacralis). On the wings dorsally there is an ear-shaped cover (facies auricularis), the ventral cover is pelvic (facies pelvina). On the vent. Transverse lines (lineae transversae) are visible again, a vascular trough passes here. The head ventrally forms a sacral promontory (promontorium). There is also a sacral canal (canalis sacralis).

The tail section of the spine is the most variable in terms of the number of vertebrae, which are 20-23 in dogs, 20-25 in pigs, 18-20 in cattle, and 18-20 in horses. In the structure of the caudal vertebrae (vertebrae caudales (coccygeae)) there is a gradual reduction of the arc. On the ventral side, from the 2nd to the 13th, hemal processes (processus hemalis) are well developed.

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 notochord throughout their lives, but they already have the beginnings of vertebrae (paired cartilaginous formations located above and below the chord): the upper arches 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; D - 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 of the 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 loach-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 elongation 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, additional bones of the wrist, tarsus, additional 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 hip joint, 6 - ilium, 7 - pubic bone, 8 - ischium, 9 - femur, 10 - sacral vertebra.