Comparative features of the axial skeleton of various vertebrate species

Different kinds of mammals in the process of evolutionary development have occupied their ecological niches, which differ, among other things, by the conditions of interactions of their organism with the gravitational field of the Earth. That is why, in the course of evolution, the axial skeleton of vertebrates underwent significant changes. Phylogenetically, the original form of the spinal column is the chorda (spinal cord), a cellular strain of entomedesoderm origin, which in the overwhelming number of vertebrates and in humans is replaced by skeletal elements. As a permanent organ of the chord, there are some lower vertebrates. In most vertebrates in the adult state, the chord remains inside the vertebrae (in fish), in vertebral bodies (amphibians) and in the form of a gelatinous nucleus (in mammals). The axial skeleton in ontogenesis passes through three stages of development:

• chordal plates (the rudiment of a string);
• partial replacement of its cartilaginous elements;
• the appearance of a bone axial skeleton.

Thus, in skeletonless skeletons, the skeleton is represented by a chord and numerous rods of dense jelly tissue, forming the skeleton of unpaired fins and the support of the gill apparatus. In the lancelet, the vertebrae consist of an almost fibrous cell mass. In the Cyclostrida, the chord is preserved throughout life, but the vertebrae appear, representing small paired cartilaginous structures evenly distributed over the chord. They are called upper arcs. In primitive fish, in addition to the upper arcs, lower arches appear, and in higher fish, the body of the vertebrae. Vertebral bodies in most fish and animals of higher classes are formed from the tissues surrounding the chord, as well as from the bases of the arches. With the bodies of the vertebrae, the upper and lower arcs coalesce. The ends of the upper arcs coalesce, forming a channel in which the spinal cord is located. On the lower arches there are processes, to which the ribs are attached.

Remains of the chord remain in the fish between the bodies of the vertebrae. The fish distinguish two parts of the vertebral column: trunk and tail. The function of the first - the maintenance of internal organs, the second - participation in the movement of the body.

The vertebral body developed in different groups of vertebrates irrespective of the chord. The bony body of the vertebra develops in the connective tissue first in the form of a thin cylinder. In cephalic and bipedal bodies, the vertebrae develop immediately as calcareous deposits of annular shape around the chord.

Phylogenetically connective tissue inner skeleton is replaced by cartilaginous, and cartilaginous - bone. During ontogenetic development, this sequence is repeated. Further changes in the spinal column depend on the development of the musculature and axial skeleton when the body moves. The vertebral column of an adult retains traces of the path of development that has been traveled.

In the adult person, with respect to the spinal column, specific features of the adaptive nature associated with the vertical position of the body are observed. When erect, the heaviness of the head affects the vertebral column, and the weakly developed facial does not require strong occipital muscles. Therefore, the occipital mound 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 hands and legs in connection with the straight-legged movement. The forelimbs of the animals, like the hind legs, are the support for the whole body and are organs of motion, so there is no sharp difference in their structure. Bones of the fore and hind limbs of animals are large and massive, their movements are equally monotonous. The finitude of the animal is not at all capable of the diverse, rapid, dexterous movements that are characteristic of the human hand.

The presence of flexures of the vertebral column in man (cervical and lumbar lordosis, thoracic and sacral-copticus and cervical kyphosis) is associated with maintaining the balance and moving the center of mass of the body with the vertical position of the body. Animals have no such bends.

A person in the structure of the vertebral column (five divisions, 33-34 vertebra) occupies a certain place among mammals. Being located one after another, the vertebrae form, as it were, two pillars - the anterior one, built at the expense of the vertebral bodies, and the posterior one, formed from the arches and intervertebral joints. In humans, the head is well balanced, and in quadruped mammals it is suspended on ligaments and muscles, beginning mainly on the cervical vertebrae and the spinous processes of the pectorals. In humans, the cervical spine consists of 7 vertebrae. With the exception of the first two, they are characterized by small, low bodies that gradually expand towards the latter. In other mammals, they are extremely massive and gradually shorten downward, which is related to the position of the head. A feature of the cervical vertebrae of a person is a bifurcated spinous process. From the general type of cervical vertebrae differ: atlant, which does not have a body and a spinous process. A characteristic feature of the vertebra C ₇ epistrophe (axial vertebra) is the presence of a directed vertically upward from the body of the vertebra of the tooth, around which, as around the axis, the atlas rotates along with the skull. The seventh cervical vertebra is distinguished by a long and undiluted spinous process, which is easily probed through the skin, and therefore is called a protruding. In addition, it has long transverse processes, and its transverse apertures are very small.

The thoracic department of the human spine consists of 12 vertebrae. The cases of the presence of a person and the 13th rib are described. Twelve pairs of ribs connect all the parts of the skeleton of the thorax to a relatively rigid system, with articular rib surfaces located on the mating lateral surfaces of two adjacent vertebrae and the intervertebral disc. Intervertebral discs in the thoracic part are covered with rib-vertebral 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 vertebral body. In the thoracic region, the intervertebral discs are wider than the bodies of adjacent vertebrae and somewhat protrude beyond them in the anterior and lateral parts, whereas in the posterior part it is not observed.

In the thoracic spine, the transverse processes in the adult are strongly deflected back, and in this connection the ribs protrude back almost to the level of the spinous processes. This feature of the structure, as well as an increase in the direction of the body of the vertebrae downstream, is specific only for 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. In connection with their oblique arrangement in the cervical region, the weight of the head is distributed not only to the bodies, but also to the articular processes. In mammals in the cervical region, they are far apart and extremely powerful, as well as the bodies of the cervical vertebrae. In a man in the thoracic and lumbar regions, the articular processes are located respectively in the frontal and sagittal planes. In this case, the severity of the overlying parts is distributed primarily to the bodies of the vertebrae, which contributes to an increase in their mass.

[1], [2], [3], [4], [5], [6]