Bones of the skeleton of limbs in ontogenesis

Two pairs of extremities are typical for almost all vertebrates. Thus, the fish have paired thoracic and ventral fins, which develop from the mesenchyme of the lateral folds.

In connection with the release of vertebrates from the aquatic environment on land, the conditions for their existence changed, which led to a significant restructuring of the organism. The terrestrial animals formed anterior and posterior limbs, the skeleton of which was constructed in the form of bone levers, consisting of several links and allowing them to move along the ground. Belts of the limbs are in rudimentary form already in fish, but they reach their greatest development in terrestrial species since amphibians. By means of belts, the limbs are connected to the trunk. The most primitive form of the skeleton of the shoulder girdle can be observed in sharks, in which it consists of dorsal and ventral cartilaginous arches, fused together closer to the ventral side of the body. From the place of fusion of these arches, on each side a free part of the fin departs. A blade is formed from the dorsal cartilaginous arch of the primitive humeral girdle both in higher fish and in terrestrial vertebrates. A joint fossa is formed at the scapula for articulation with a skeleton of a free limb section.

From the ventral cartilaginous arch there is a coracoid, which in amphibians, reptiles and birds is fused with the sternum. In viviparous mammals, the coracoid is partially reduced and grows to the scapula in the form of a beak-shaped process. From this same rudiment another process develops, called "procoracoid", on the basis of which the covering bone, the clavicle, is then formed. Its medial end of the collarbone is connected with the sternum, and lateral - with a scapula. These bones are developed in mammals, in which a free limb section can move around all axes. In animals where movement during running and swimming takes place only around one axis (ungulates, predators and cetaceans), the clavicles are reduced.

The pelvic girdle of the fish is in its infancy and does not connect with the bell-shaped column, since they do not have fish of the sacrum. In sharks, the pelvic girdle is represented by a dorsal and ventral cartilaginous arch. From the place of their adhesion, the rear fins depart from each other. The dorsal cartilaginous arch of the pelvic girdle in terrestrial animals develops into the iliac bone. From the ventral cartilaginous arch there are sciatic and pubic bones corresponding to the coracoid and prokoroid of the shoulder girdle. These three rudiments of the hip bone are connected to each other in the place where the articular fossa is formed for articulation with the free section of the hindlimb. In mammals, with age, all three bones merge into one pelvic bone, and the cartilage between them completely disappears. Both pelvic bones in higher vertebrates, especially in monkeys and humans, are connected by their ventral ends, and a sacrum is inserted from the dorsal side between them. Thus, the bone ring - the pelvis is formed. In animals, the pelvis is a support for the hind limbs, and in humans - for the lower extremities due to its vertical position. In humans, the iliac bones expand considerably to the lateral sides, taking on the function of supporting the internal organs of the abdominal cavity.

The skeleton of a free limb section in fish consists of a series of cartilaginous or bone segments arranged in the form of rays and creating a solid foundation for the fins. In the skeleton of the limbs of terrestrial vertebrates, the number of rays was reduced to five. The skeleton of anterior and posterior extremities in animals, upper and lower extremities in man has a general plan of structure and is represented by three links that follow each other: the proximal link (humerus and pelvic bones), the middle link (ray and ulna, pectoral and fibular bones) and the distal link (brush, stop). The brush and foot in the proximal part consist of small bones, while in the distal part they represent five free rays, which have received the names of the fingers. All these bone fragments are homologous on both extremities.

In connection with the conditions for the existence of terrestrial vertebrates in the distal link of the limbs, individual bone elements fused into one bone or were subjected to reduction. Less frequent development of additional pits, most of which are sesamoid (patella, pea, etc.). In terrestrial animals, not only the anatomy of the limbs has changed - their formulation has also changed. Thus, in amphibians and reptiles, the proximal part of the free parts of both pairs of limbs is located at right angles to the trunk, and the bend between the proximal and middle links also forms an angle open in the medial direction. In the higher forms of vertebrates, the free region is located in the sagittal plane with respect to the trunk, with the proximal limb of the forelimb turning posteriorly, and the proximal limb of the hind limb anterior. As a result, the elbow joint is turned back, and the knee joint of the lower limb is directed forward.

In the process of further development of vertebrates, the forelimbs began to adapt to a more complex function than the rear ones. In connection with this, their structure also changed. An example is the wing in birds as an aircraft. Climbing mammals developed a grasping limb with the opposition of the thumb to the rest. This function is possessed by all four limbs of the monkey.

The person, the only one of all vertebrates, acquired an upright position, began to rely only on the hindlimb (lower) limbs. The human forelimbs, which became upper in connection with the vertical position, were completely freed from the function of movement of the body in space, which enabled them to make sometimes very subtle movements. In connection with this, the bones of the arm from the bones of the foot are distinguished by a greater ease and subtle structure. They are connected with each other by moving joints. Freedom of movement of the upper limb in humans also depends on the presence of the clavicle, which sets the free upper limb aside. The man's brush has adapted to working activity, namely: the wrist bones are small, movably connected to each other; fingers extended and became mobile; the thumb is located almost at right angles to the bones of the pastern, is very mobile and contrasts with all the other fingers, which ensures the grasping function of the hand when performing complex work.

The lower limb of a person performs the function of supporting, holding the body in an upright position and moving it in space. In connection with this, the bones of the lower limb are massive, the joints between the individual links are less mobile than in the upper limb. The different function of the upper and lower extremities in a person most affected the distal link - the hand and the foot.

The brush develops and is perfected as an organ of labor. The foot serves to support the body, it has all its weight. The toes do not play a significant role in the support, they are greatly shortened. The thumb is located in a row with the other fingers and is not very mobile.

The foot is a mechanically complex vaulted formation, so it serves as a springing support, on which depends the smoothing of shocks and shocks during walking, running and jumping.

In ontogenesis in man, the rudiments of the limbs appear in the third week of embryonic life in the form of a cluster of mesenchymal cells in the lateral folds of the embryo's body resembling the fins of fish. The folds widen and form plates, giving rise to the brushes and somewhat later to the feet. In these rudiments one can not yet distinguish between the fingers; they are formed later in the form of 5 rays. The sequence of further development of the elements of the future limbs is observed in the direction from the distal limb to the proximal.

All bones of the limbs, with the exception of the clavicles, which develop on the basis of connective tissue, bypassing the stage of cartilage, undergo three stages of development. In this case, the diaphytes of all bones ossify in the uterine period, and the epiphyses and apophyses - after birth. Only a few epiphyses begin to ossify shortly before birth. In each bone, a certain number of ossification points are laid, which appear in a known order. In the diaphysis of the tubular bones, the primary ossification point appears at the end of the second and beginning of the third month of intrauterine life and grows in the direction of the proximal and distal epiphyses. Epiphyses of these bones in newborns are still cartilaginous, and secondary ossification points in them are formed after birth, during the first 5-10 years. Bony epiphyses grow to the diaphysis after 15-17 and even after 20 years. Special attention should be paid to the timing of the appearance of major ossification points in the formation of individual bones.

[1], [2], [3], [4], [5]