The vertebral column consists of a long series of irregular-shaped bones termed vertebrae, united together in various ways to form a long undulating column commonly known as the "spine".
Vertebrae are divisible into true and false. The former are characterized by the presence of a certain group of parts, some of which are absent in the latter. Moreover, true vertebrae in health are always free and separate from each other, while false ones may become joined together by bony union. Examples of the latter are seen in the bones of the sacrum and those of the coccyx or tail.
Each of the several vertebrae, from the head backward as far as the commencement of the tail, forms a ring which, when the whole are brought together, constitutes the spinal canal in which is enclosed the spinal cord.
The vertebral column contains from 50 to 54 pieces, which for convenience of description are divided into four sections, viz.: the cervical, dorsal, lumbar, and sacro-coccygeal.
The first 7 bones are the cervical vertebrae or neck-bones; beyond these are 18 dorsal vertebrae or back-bones, behind which are sometimes 5 but mostly 6 lumbar or loin bones, and beyond these are 5 sacral bones, corresponding to the croup, and 14 to 18 coccygeal or tail bones.
For the most part the vertebrae composing these several regions bear more or less resemblance to each other, but possess some special differences by which bones of one region may be distinguished from those of another.
True Vertebrae are characterized by a number of bony prominences or processes, a central canal for the accommodation of the spinal cord, a solid discoidal mass or body and an arch (neural arch). The anatomical parts of a vertebra are shown in figs. 4 and 5, Plate XXXVIII.
A conspicuous exception to this formula is presented by the first cervical vertebra, which is a simple ring of bone with two broad sloping transverse processes and a small inferior spinous process (fig. 1, Plate XXXVIII).
The superior spinous processes of the neck are very short, those of the back and loins are long, especially in the region of the withers, where they increase in length from the first to the fifth and then diminish again backward (Plate XXXVII).
The inferior spinous processes are for the most part small, and in some of the bones only exist in a very rudimentary state.
The two transverse processes, right and left, consist of irregular bony prominences varying in form and size in different parts. In all they serve for the attachment of muscles, in addition to which those of the dorsal vertebrae are also united to the ribs, with which they have a synovial articulation. In the neck an opening passes through the transverse processes of the first six vertebrae, while in the loins these processes are very long and flat, and some of them behind have synovial articulations by which they are joined together.
The oblique processes are situated on the anterior and posterior parts of the arch. They form joints with corresponding parts on the bones in front and behind them by broad synovial surfaces, the two anterior of which look upward and inward, while the two posterior look downward and outward.
The Body is the thick solid base on which the arch rests, and which forms the floor of the spinal canal. Its anterior extremity is round or convex, and fits into a corresponding hollow or concavity in the bone before it. Its posterior extremity is concave, and receives the rounded end of the vertebra which follows it. These convexities and concavities are much greater in the cervical vertebrae than in other regions, on account of which the neck is able to move with exceptional freedom in all directions. On either side, in front and behind, a small depression exists on the bodies of the dorsal vertebrae for the accommodation of the heads of the ribs, which fit in between them to form a synovial articulation.
The Neural Arch is formed by two plates of bone which spring from the upper surface of the body on either side, and unite above to form the spinal canal. In the anterior and posterior borders of the neural arch above the body of each vertebra are two notches which, with corresponding notches in the vertebrae before and behind it, form openings, termed the intervertebral foramina, through which the spinal nerves leave the spinal canal.
The False Vertebrae are those of the sacrum, the several pieces of which are firmly joined together by bony union, and the coccygeal bones, from which some of the parts above described are wanting or exist only in a rudimentary form.