Musculoskeletal system. Skeleton: definition, functions and its phylo-ontogenesis

The concept " phylogenesis"(from the Greek phyle - “clan, 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.

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

Exoskeleton performs protective function. It is inherent in lower vertebrates and is located on the body in the form of scales or shell (turtle, 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 to the internal skeleton.

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

In primitive chordates (lancelets), along with the septa, an axis appears - the notochord (cellular cord), covered with connective tissue membranes. In fish, the spine is relatively simple and consists of two sections (trunk and caudal). Them soft cartilaginous spine more functional than chordates; The spinal cord is located in the vertebral canal. The skeleton of fish is more perfect, allowing faster and more precise movements with less weight.

With the transition to a terrestrial lifestyle, 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 of lamellar bone tissue, consisting of bone plates containing ordered collagen fibers.

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

Skeleton of a mammal (left) and a fish (right)

Decoding the lancelet genome, completed in 2008, confirmed the closeness of 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. His characteristic feature- 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 determines the length of the neck. The only exceptions are two animals: the manatee has 6 of these vertebrae, and different types sloths - from 8 to 10. Giraffe cervical vertebrae very long, and in cetaceans that do not have a cervical interception, on the contrary, they are extremely short.

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

IN lumbar region from 2 to 9 vertebrae; Rudimentary ribs merge with their large transverse processes. The sacral section 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 caudal vertebrae ranges from 3 (in the gibbon) to 49 (in the long-tailed lizard).

The mobility of individual vertebrae depends on lifestyle. Thus, 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 vertebrae of the thoracic and lumbar regions are less mobile in large, fast-moving animals. In mammals that move on hind legs(kangaroos, jerboas, jumpers), the largest vertebrae are located at the base of the tail and sacrum, and then their size successively 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 the forelimbs are attached to them.

In birds, the forelimbs (wings) are adapted for flight, and the hind limbs are adapted for moving on the ground. A peculiar feature of the skeleton is the pneumaticity of the bones: they are lighter because they contain air. Bird bones 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.

Skeleton(from the Greek "skeleton" - dried) are structures of various structures and origins that ensure the preservation of the shape of the animal's body, as well as support and protection for internal organs. In addition, they are attached to individual components of the skeleton. muscles, ensuring the movement of the animal - so the skeleton is an important functional subdivision of the musculoskeletal system. Vertebrates, unlike most invertebrates, have endoskeleton- i.e. their supporting structures are located not on the surface, but in the deep parts of the body.

The prototype of the vertebrate skeleton - and also the only skeletal structure in lower chordates - is chord, a dense cord of cells of mesodermal origin, stretching along the dorsal (dorsal) side through the entire body, from head to tail. In higher chordates - vertebrates- the notochord is preserved only at the embryonic stage of development, being replaced in adulthood by cartilaginous and bone tissues formed in ontogenesis from mesenchyme, i.e. embryonic connective tissue is predominantly of mesodermal origin. Initially, skeletal elements are formed from cartilage; however, now a cartilaginous skeleton is observed only in lower groups of vertebrates ( lampreys, hagfish, cartilaginous fish and some others). In higher vertebrates, cartilaginous structures are observed mainly in the embryonic stage of development and in childhood; in adulthood, their skeleton is built mostly from bones.

Anatomically, the vertebrate skeleton is formed by many elements that have different structures, shapes, origins and locations in the animal’s body. These skeletal elements (cartilage or bones) are connected to each other or immovable ( synarthrosis) or movable ( joints) joints; the latter option ensures the movement of body parts relative to each other and the entire body of the animal in the surrounding space. Despite all the diversity, the skeletal elements of various groups of vertebrates can be combined into several sections.

Integumentary skeleton

The integumentary skeleton is a collection of bone elements located in the skin of the animal; these elements are initially formed from bone tissue and do not have a cartilaginous stage of development. The skin of modern vertebrates usually does not contain any bone elements, but in many extinct forms the body was partially or completely enclosed in a bony shell; in addition, some bones are of integumentary origin skulls And limb belts.

Modern lampreys And mikisny do not have any bony shell, but many ancient aquatic vertebrates (for example, armored fish) were completely clad in powerful armor; The overwhelming majority of modern fish also have a protective layer on top of the skin made of bony scales of various shapes and structures; the covering bones also include elements of the operculum.

Terrestrial four-legged vertebrates initially also had a complete bone cover of plates and scales; subsequently, some of its components became part of the skull, jaws, and limb girdles, while others were lost. The skin of these vertebrates, however, retained the ability to form bone, so that some of their representatives secondarily acquired protective scales or plates - for example, abdominal ribs crocodiles, shell turtles And armadillos.

Internal skeleton

Bird skeleton

Spine

Spinal column, formed by many vertebrae, - essential element so-called axial skeleton , historically formed around the notochord, although the notochord itself is reduced in adulthood, remaining only in fish, primitive amphibians And reptiles, being strongly compressed within the vertebrae and expanding between them; in most terrestrial vertebrates, the remains of the notochord are only gelatinous formations in intervertebral discs . Individual vertebrae have different structures in different groups of vertebrates; in addition, within the same organism, the vertebrae are also heterogeneous, which makes it possible to distinguish several sections of the spine. The spine of fish is most simply structured - only the trunk and caudal sections are clearly distinguished; in the course of further evolution, the thoracic, cervical, lumbar and sacral regions became separated; Each group of vertebrates has its own special set of spinal sections.

The axial skeleton includes ribs, first appearing in cartilaginous fish and representing elongated cartilaginous or bone formations, serving mainly for muscle attachment; Different groups of vertebrates have ribs of different shapes, sizes and origins, connected to the vertebrae of one or more parts of the spine. On the ventral (ventral) side, the ribs can join sternum, thus forming chest.

Scull

Skeleton of the head - scull- is a very complex formation, consisting of many cartilaginous or bone elements with different structures and origins: here there is a combination of both internal and integumentary bones fused with them. IN general outline The vertebrate skull consists of four components:

brain box- in fact, it is a continuation of the axial skeleton, formed along the back, lower and lateral sides of the brain from the internal and partially integumentary bones. The occipital region also contains foramen magnum, through which the spinal cord passes, and also condyles to connect to the first vertebra.
skull roof- bone elements covering the brain from above, in front and from the sides, as well as forming the structures of the nose, eye sockets, temporal region, upper jaw, and formed exclusively by the integumentary bones.
palatal complex- elements that form the primary and secondary palate and are formed by the internal and integumentary bones.
visceral skeleton- cartilaginous or bone elements that initially form around oral cavity and pharynx, but originating from the mesenchyme of endodermal origin. The lower chordates present gill arches, the front of which are transformed into jaws; in the higher ones they are supplemented by the integumentary bones of the lower jaw and sublingual area, the remains of the former gill arches are transformed into the bones of the middle ear or into cartilage not related to the skeleton itself larynx.

Skeleton of limb belts

Limb belts- these are cartilaginous or bone formations designed to connect the limbs themselves to the body. Accordingly with the limbs, they distinguish shoulder girdle, or the belt of the forelimbs, and pelvic girdle, or the belt of the hind limbs. The composition and structure of the limb girdles vary among different groups of vertebrates, but some general patterns are observed.

shoulder girdle consists of two parts - integumentary and internal origin. The integumentary ones include collarbone and some other bones that provide connection between the forelimb and the spine, and in fish, also with the skull. Internal bones shoulder girdle present in higher vertebrates spatula- a bone directly connected to the forelimb and used for muscle attachment.
pelvic girdle- a purely endoskeletal formation that serves to attach the muscles of the hind limb. In fish, the pelvic girdle is simple element, not connected in any way with the axial skeleton; in terrestrial vertebrates, on the contrary, it is attached to the spine and consists of clearly distinguishable three pairs of bones.

Limb skeleton

Free limbs vertebrates that serve as a means of transportation have some variations among different groups. So, ray-finned fish have paired fins(thoracic and abdominal), built according to the fold principle; these limbs have practically no internal skeleton, being supported by rays of integumentary origin. Fins of the Ancients lobe-finned fish, on the contrary, demonstrate a typical three-segmented structure, in which the segment closest to the body is formed by a single element, the middle segment by two elements, and the distal segment by many small bones arranged in the form of a blade. Terrestrial vertebrates inherit a similar pattern, with the third (distal) segment remaining general case only five rays - this is how a typical five-fingered limb is formed, consisting of shoulder, forearms And brushes(for the front) or from hips, shins And feet(for the back).

What are the functions musculoskeletal system?

The musculoskeletal system performs the functions of support, maintaining a certain shape, protecting organs from damage, and movement.

Why does the body need a musculoskeletal system?

The musculoskeletal system is necessary for the body to maintain vital functions. It is responsible for maintaining shape and protecting the body. The most important role of the musculoskeletal system is movement. Movement helps the body in choosing habitats, searching for food and shelter. All functions of this system are vital for living organisms.

Questions

1. What underlies the evolutionary changes in the musculoskeletal system?

Changes in the musculoskeletal system had to fully ensure all evolutionary changes in the body. Evolution has changed the appearance of animals. In order to survive, it was necessary to search for food more actively, to hide or defend better from enemies, and to move faster.

2. Which animals have an exoskeleton?

The exoskeleton is characteristic of arthropods.

3. Which vertebrates do not have a bony skeleton?

Lancelets and cartilaginous fish do not have a bony skeleton.

4. What does the similar structure of the skeletons of different vertebrates indicate?

The similar structure of the skeletons of different vertebrates indicates the unity of origin of living organisms and confirms the evolutionary theory.

5. What conclusion can be drawn after becoming familiar with the general functions of the musculoskeletal system in all animal organisms?

The musculoskeletal system in all animal organisms performs three main functions - supporting, protective, and motor.

6. What changes in the structure of protozoa led to an increase in the speed of their movement?

The first supporting structure of animals - the cell membrane - allowed the body to increase the speed of movement due to flagella and cilia (outgrowths on the membrane)

Tasks

Prove that the complication of the amphibian skeleton is associated with changes in the habitat.

The skeleton of amphibians, like other vertebrates, consists of the following sections: the skeleton of the head, torso, girdles of the limbs and free limbs. Amphibians have significantly fewer bones than fish: many bones are fused, and in some places cartilage is preserved. The skeleton is lighter than that of fish, which is important for terrestrial existence. Wide flat skull and upper jaws represent a single entity. Very mobile lower jaw. The skull is movably articulated to the spine, which plays an important role in terrestrial food production. The spine of amphibians has more sections than that of fish. It consists of the cervical (one vertebra), trunk (seven vertebrae), sacral (one vertebra) and caudal sections. The tail of a frog consists of a single tail bone, while that of tailed amphibians consists of separate vertebrae. The skeleton of the free limbs of amphibians, unlike fish, is complex. The skeleton of the forelimb consists of the shoulder, forearm, wrist, metacarpus and phalanges of the fingers; hind limb - thigh, tibia, tarsus, metatarsus and phalanges. The complex structure of the limbs allows amphibians to move in both aquatic and terrestrial environments.

Spinal column: structure, development, specific features

According to its development spinal column(columna vertebralis) is formed around spinal cord, forming a bone receptacle for it. In addition to protecting the spinal cord, the spinal column also performs other functions in the body. important functions: 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 cavities and pelvis.

Spinal column(columna vertebralis) consists of individual 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 between the arch and the body a vertebral foramen (foramen vertebrae) is formed. All vertebral foramina 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 nerves and blood vessels. Along the edges of the arches protrude the cranial and caudal articular processes (processus articularis cranialis et caudalis), which serve to articulate the vertebrae with each other. The spinous process (processus spinosus) protrudes - anchoring muscles and ligaments.

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

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

· 1st(atlas - atlas), consists of two arches (arcus dorsalis et ventralis), on them, respectively, there are tubercles (tuberculum dorsale et ventrale). The transverse processes form the wings of the atlas (ala atlantis). Under the wing there is a fossa atlas (fossa atlantis), on the wings there are two pairs of openings for blood vessels and nerves - alar (foramen alare) and 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 vertebral head and a ridge (crista dorsalis) instead of the spinous process, also a single transverse process (processus transversus).

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

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

In the thoracic region of vertebrates, cattle and dogs have 13 vertebrae, pigs have 14-17, and horses have 18. The thoracic vertebrae (vertebrae thoracicae) together with the ribs and sternum form the chest. The vertebrae of this section 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 inclined back towards the tail. Spinous processes vertebrae from the 2nd to the 9th form the base of the withers (regio interscapularis). The spinous process of the 13th (12th in a pig, 16th in a horse, 11th in a dog) vertebra stands vertically - diaphragmatic. On the transverse processes (processus transversus) are located mastoid processes(processus mamillaris).

IN lumbar region The spine in cattle and horses has 6 vertebrae, in pigs and dogs there are 7. Lumbar vertebrae(vertebrae lumbales), characterized by the presence of long, flat transverse processes and well-developed articular processes. (In the domestic bull:) vertebral bodies with a waist-shaped interception, transverse processes with sharp jagged edges and curved forward towards the head. The spinous processes stand vertically. The cranial articular processes form semicylindrical bushings, and the caudal ones form the same blocks.

IN 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), and there are no interaricular foramina. The 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, the auricular part (facies auricularis) is located dorsally, and the ventral part is the pelvic part (facies pelvina). On the vent. Transverse lines (lineae transversae) are visible, and the vascular groove runs here. The head ventrally forms the promontory of the sacrum (promontorium). There is also a sacral canal (canalis sacralis).

The caudal spine is the most variable in the number of vertebrae, of which there 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)), a gradual reduction of the arch is observed. On the ventral side from 2 to 13, hemal processes (processus hemalis) are well developed.

Musculoskeletal system ensures movement and preservation of the animal’s body position in space, forms external form body and participates in metabolic processes. It accounts for about 60% of the body weight of an adult animal.
Conventionally, the musculoskeletal system is divided into passive and active parts. The passive part includes bones and their connections, on which the nature of the mobility of bone levers and links of the animal’s body depends (15%). The active part is skeletal muscles and their auxiliary devices, thanks to the contractions of which, the bones of the skeleton are set in motion (45%). Both active and passive parts have common origin(mesoderm) and are closely interconnected.

Functions of the movement apparatus:

1) Physical activity is a manifestation of the vital activity of an organism, it is what distinguishes animal organisms from plant organisms and determines the emergence of a wide variety of methods 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 occurred under the influence of the gravitational field of the Earth, its size and shape in vertebrates differ in significant diversity, which is explained different conditions their habitats (terrestrial, terrestrial-woody, airy, aquatic).
3) In addition, the movement apparatus provides a number of vital functions of the body: searching and capturing food; attack and active protection; carries out respiratory function lungs (respiratory motility); Helps the heart move blood and lymph through the vessels (“peripheral heart”).
4) In warm-blooded animals (birds and mammals), the movement apparatus ensures the preservation constant temperature bodies;
The functions of the movement apparatus are provided by the nervous and cardiovascular systems, respiratory, digestive and urinary organs, skin, glands internal secretion. Since the development of the movement apparatus is inextricably linked with the development nervous system, then when these connections are disrupted, first paresis occurs, and then paralysis of the movement apparatus (the animal cannot move). When decreasing physical activity a violation occurs metabolic processes and atrophy of muscle and bone tissue.
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 occur. The energy of elastic deformations in bones is converted into piezoelectric energy, and in muscles into thermal energy. 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 movement apparatus is the skeleton. Skeleton (Greek sceletos - dried, 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 includes about 200-300 bones (Horse -207), which are connected to each other using connective, cartilage or bone tissue. The skeletal mass of an adult animal is 15%.
All functions of the skeleton can be divided into two large groups: mechanical and biological. Mechanical functions include: protective, support, locomotor, spring, anti-gravity, and biological functions include metabolism and hematopoiesis (hemocytopoiesis).
1) The protective function is that the skeleton forms the walls of the body cavities in which vital important organs. For example, the cranial cavity contains the brain, the chest contains the heart and lungs, and the pelvic cavity contains the genitourinary organs.
2) The supporting function is that the skeleton provides a support for muscles and internal organs, which, when 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 driven by 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 shocks (cartilaginous pads, etc.).
5) The anti-gravity function is manifested in the fact that the skeleton creates support for the stability of the body rising above the ground.
6) Participation in metabolism, especially 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 internal environment body (homeostasis).
8) Participation in hemocytopoiesis. Located in the bone marrow cavities, red bone marrow produces blood cells. Weight bone marrow in relation to bone mass in adult animals is approximately 40-45%.

The spinal column is divided into 5 sections: cervical, thoracic, lumbar, sacral and caudal. Cervical region consists of cervical vertebrae (v.cervicalis); thoracic region - from the thoracic vertebrae (v.thoracica), ribs (costa) and sternum (sternum); lumbar - from the lumbar vertebrae (v.lumbalis); sacrum - from the sacrum bone (os sacrum); caudal - from the caudal vertebrae (v.caudalis). The most complete structure has the thoracic region of the body, where there are thoracic vertebrae, ribs, sternum, which together form the chest (thorax), in which the heart, lungs, and mediastinal organs are located. The tail region is the least developed in terrestrial animals, 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 laws of body structure, which ensure the mobility of the animal. These include:
1) Bipolarity (uniaxiality) is expressed in the fact that all parts of the axial skeleton are located on the same axis of the body, with the skull on the cranial pole and the tail on the opposite pole. The sign of uniaxiality allows us 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, like the torso, 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 (antimerism) makes it possible to distinguish lateral (lateral, external) and medial (internal) directions on the animal’s body.
3) Segmentation (metamerism) lies in the fact that the body can be divided by segmental planes into a certain number of relatively identical metamers - segments. Metameres follow an axis from front to back. On the skeleton, such metameres are vertebrae with ribs.
4) Tetrapodium is the presence of 4 limbs (2 thoracic and 2 pelvic)
5) And the last regularity is, due to the force of gravity, the location in the spinal canal of the neural tube, and below it the intestinal tube with all its derivatives. In this regard, the dorsal direction is marked on the body - towards the back and the ventral direction - towards the abdomen.

The peripheral skeleton is represented by two pairs of limbs: pectoral and pelvic. In the skeleton of the limbs there is only one pattern - bilaterality (antimerism). The limbs are paired, there are left and right limbs. The remaining elements are asymmetrical. On the limbs there are girdles (thoracic and pelvic) and a skeleton of free limbs.

Skeletal phylogeny

In vertebrate phylogenesis, the skeleton develops in two directions: external and internal.
The exoskeleton performs a protective function, is characteristic of lower vertebrates and is located on the body in the form of scales or shell (turtle, 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 and, located under the skin, connected 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 skull bones are injured, they are forced to be replaced with artificial plates.
The internal skeleton performs mainly a supporting function. During development, under the influence of biomechanical load, it constantly changes. If we consider invertebrate animals, then their internal skeleton has the form of partitions to which muscles are attached.
In primitive chordates (lancelet), along with the septa, an axis appears - the notochord (cellular cord), covered with connective tissue membranes.
In cartilaginous fish (sharks, rays), cartilaginous arches are formed segmentally around the notochord, which later form vertebrae. The cartilaginous vertebrae, connecting to each other, form the spinal column, and the ribs are attached to it ventrally. Thus, the chord remains in the form of nuclei pulposus between the vertebral bodies. The skull is formed at the cranial end of the body and, together with the vertebral column, participates in the formation of the axial skeleton. Subsequently, the cartilaginous skeleton is replaced by a bone one, less flexible, but more durable.
In bony fishes, the axial skeleton is built from stronger, coarse-fibrous bone tissue, which is characterized by the presence of mineral salts and a random arrangement of collagen (ossein) fibers in the amorphous component.
With the transition of animals to a terrestrial lifestyle, amphibians form a new part of the skeleton - the skeleton of the limbs. As a result of this, in terrestrial animals, in addition to the axial skeleton, a peripheral skeleton (the skeleton of the limbs) is also formed. In amphibians, as well 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 of lamellar bone tissue, consisting of bone plates containing collagen (ossein) fibers arranged in an orderly manner.
Thus, the internal skeleton of vertebrates goes 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).

Skeletal ontogeny

In accordance with the basic biogenetic law of Baer and E. Haeckel, in ontogenesis the skeleton also goes through three stages of development: 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 the membranous skeleton. Then a notochord appears in the embryo, and around it, first a cartilaginous, and later a bony spinal column and skull, and then 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 the body weight. Each cartilage has the shape of a future bone and is covered with perichondrium (a dense connective tissue membrane). 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 (enchondral ossification). In place of the cartilage, coarse fibrous bone tissue is formed. As a result of this, in fruits the skeleton is built of coarse fibrous bone tissue.
Only in the neonatal period is coarse fibrous bone tissue replaced by more advanced lamellar bone tissue. During this period, special attention to newborns is required, since their skeleton is not yet strong. As for the chord, its remains are located in the center intervertebral discs in the form of nucleus pulposus. Special attention During this period, it is necessary to pay attention to the integumentary bones of the skull (occipital, parietal and temporal), since they bypass the cartilaginous stage. Between them in ontogenesis, significant connective tissue spaces called fontanelles (fonticulus) are formed; only in old age do they completely undergo ossification (endesmal ossification).