Comparative features of the axial skeleton of different vertebrate species

Various species of mammals in the process of evolutionary development have occupied their ecological niches, which differ, among other things, in the conditions of interaction of their organism with the gravitational field of the Earth. This is why the axial skeleton of vertebrates has undergone significant changes in the process of evolution. The phylogenetically original form of the spinal column is the notochord (spinal cord) - a cellular strand of endomesodermal origin, which in the overwhelming majority of vertebrates and in humans is replaced by skeletal elements. As a permanent organ, the notochord exists in some lower vertebrates. In most vertebrates, in adulthood, the notochord is retained inside the vertebrae (in fish), in the bodies of the vertebrae (in amphibians) and in the form of a gelatinous nucleus (in mammals). The axial skeleton in ontogenesis goes through three stages of development:

• chordal plates (rudiment of a string);
• partial replacement of it with cartilaginous elements;
• the emergence of the axial skeleton.

Thus, in acrania, the skeleton is represented by a notochord and numerous rods of dense gelatinous tissue, forming the skeleton of unpaired fins and the support of the gill apparatus. In the lancelet, the vertebrae consist of an almost fiberless cellular mass. In cyclostomes, the notochord is preserved throughout life, but vertebral rudiments appear, which are small paired cartilaginous formations evenly located above the notochord. They are called the upper arches. In primitive fish, in addition to the upper arches, lower arches appear, and in higher fish - the bodies of the vertebrae. The bodies of the vertebrae in most fish and animals of higher classes are formed from the tissues surrounding the notochord, as well as from the bases of the arches. The upper and lower arches grow together with the bodies of the vertebrae. The ends of the upper arches grow together, forming a canal in which the spinal cord is located. On the lower arches, processes appear to which the ribs are attached.

The remains of the chord are preserved in fish between the bodies of the vertebrae. Fish have two sections of the spinal column: trunk and tail. The function of the first is to support the internal organs, the second is to participate in the movement of the body.

The vertebral body developed in various groups of vertebrates independently of the notochord. The bony body of the vertebra develops in the connective tissue first as a thin cylinder. In whole-headed and dibreathing animals, the vertebral bodies develop immediately as calcareous ring-shaped deposits around the notochord.

Phylogenetically, the connective tissue internal skeleton is replaced by cartilaginous, and cartilaginous by bone. During ontogenetic development, this sequence is repeated. Further changes in the spinal column depend on the development of muscles and the axial skeleton during body movements. The spinal column of an adult retains traces of the developmental path taken.

In an adult, the spinal column exhibits specific adaptive features related to the vertical position of the body. When walking upright, the weight of the head affects the spinal column, and the poorly developed facial region does not require strong occipital muscles. Therefore, the occipital protuberance and other elevations and irregularities on the skull are poorly developed in humans.

The difference in the structure of the upper and lower limbs of a person is due to the difference in the functions of the arms and legs in connection with upright walking. The forelimbs of animals, like the hind limbs, serve as support for the entire body and are organs of movement, so there is no sharp difference in their structure. The bones of the front and hind limbs of animals are large and massive, their movements are equally monotonous. The limb of an animal is not at all capable of varied, fast, dexterous movements, which are characteristic of the human hand.

The presence of curves in the human spine (cervical and lumbar lordosis, thoracic and sacrococcygeal kyphosis) is associated with maintaining balance and moving the body's center of mass in a vertical position. Animals do not have such curves.

In terms of the structure of the spinal column (five sections, 33-34 vertebrae), humans occupy a certain place among mammals. Located one after the other, the vertebrae form two columns - the front, built by the bodies of the vertebrae, and the back, formed by the arches and intervertebral joints. In humans, the head is well balanced, and in four-legged mammals it is suspended by ligaments and muscles that begin mainly on the cervical vertebrae and the spinous processes of the thoracic vertebrae. In humans, the cervical section of the spinal column consists of 7 vertebrae. With the exception of the first two, they are characterized by small low bodies that gradually expand towards the last G. In other mammals, they are extremely massive and gradually shorten downwards, which is due to the position of the head. A feature of the human cervical vertebrae is the bifurcated spinous process. The following differ from the general type of cervical vertebrae: the atlas, which has no body and spinous process. A characteristic feature of the C _7th epistropheus vertebra (axial vertebra) is the presence of a tooth directed vertically upward from the body of the vertebra, around which, like around an axis, the atlas rotates together with the skull. The seventh cervical vertebra is distinguished by a long and unbifurcated spinous process, which is easily palpated through the skin, and therefore is called protruding. In addition, it has long transverse processes, and its transverse openings are very small.

The thoracic spine of a human consists of 12 vertebrae. There are cases of humans having a 13th rib. Twelve pairs of ribs connect all sections of the thoracic skeleton into a relatively rigid system, with the articular surfaces of the ribs located on the articulating lateral surfaces of two adjacent vertebrae and the intervertebral disc. The intervertebral discs in the thoracic spine are covered from the side by the costovertebral joints. The exception is the level of the 12th vertebra, and sometimes the 11th, where the articulation occurs not at the level of the disc, but directly on the body of the vertebra. In the thoracic spine, the intervertebral discs are wider than the bodies of adjacent vertebrae and protrude somewhat beyond their limits in the anterior and lateral parts, while this is not observed in the posterior part.

In the thoracic spine, the transverse processes of an adult human are strongly deflected backwards, and in connection with this, the ribs protrude backwards almost to the level of the spinous processes. This structural feature, as well as the downward increase in the bodies of the vertebrae, is specific only to humans and is an adaptation to the vertical position. This is not observed in animals.

The position of the articular processes is not the same in different parts of the spinal column. Due to their oblique position in the cervical region, the weight of the head is distributed not only on the bodies, but also on the articular processes. In mammals, in the cervical region, they are located far from each other and are extremely powerfully developed, as are the bodies of the cervical vertebrae. In humans, in the thoracic and lumbar regions, the articular processes are located in the frontal and sagittal planes, respectively. In this case, the weight of the overlying parts is distributed mainly on the bodies of the vertebrae, which contributes to an increase in their mass.

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