This portion of the skeleton consists of the backbone or vertebral column, on the top of which the head is supported, and the chest or thorax.
The Vertebral Column is composed origin ally of 32 separate pieces, each piece being called a vertebra. In the adult state the sepa rate pieces number only 26, several having be come fused together. The separate pieces are arranged one on the top of the other, cushions of gristle being interposed between each (Fig.
14, g, g), which also help to unite them, while the union is completed by partially movable joints and by strong fibrous bands called liga ments. The column so ar ranged presents two for ward curves, the first (a) in the neck, the second (b) at the lower part of the back, and there are two corresponding backward curves. The first seven vertebrae (i to 7) occupy the region of the neck, and are, therefore, called cer vical (Latin, cervix = the neck); twelve (from 7 to 19) are the supports from which spring the ribs, and constitute the main portion of the back, being called accordingly dorsal ; the next five (io to 24) are called lumbar, in the region called the "small of the back." Following these there come five bones, separate in early life, but united in the adult into one piece, called the sacrum (s), which forms with the haunch bones on each side a large basin shaped cavity—the pelvis. Attached to the end of the sacrum is a small pointed bony mass, containing four vertebrae, originally sepa rate and of a rudimentary kind, which corre sponds with the bony part of the tail in other animals, and is called coccyx (c).
Each vertebra consists of a body (Fig. 15, a) from which two arches pass backwards (c, c), which meet in the middle line (b) and are pro longed into a projection or process—the spin ous process (d). The arches inclose a ring (h). Projecting upwards and downwards from the sides of the ring are processes—two above by which the vertebra is united to the one above it, and two below for union with the one below it. These are called articulating processes (g, Fig. 15). Just about the position of these, there pass transversely outwards other two projections, one on each side, called the trans verse processes (e). Now when the vertebral are in position the spinous processes are in line. They project backwards— the bodies being in front (see Fig. 14)—and give the ir regular feeling that is experienced when one passes the hand down the centre of a person's back. At the same time the rings are all one above the other, and so form a canal (the spinal canal) in which the spinal cord lies, and by the bony walls of which it is protected. The verte
brae differ slightly according as they belong to the cervical, dorsal, or lumbar region. The cervical vertebrae (Fig. 16) have a canal (f) in the transverse process for the passage of an artery, and their spinous processes (d) are split at the extremity. The bodies of the lumbar vertebrae are more massive than those of the dorsal. Their transverse and articulating pro cesses are more marked (see Fig.
17, references the same as in Fig.
15), and their spinous processes are hatchet-shaped and point straight backwards, while the spinous pro cesses of the dorsal vertebra overlap one another, and the transverse processes have little polished surfaces for uniting with the ribs (Fig. 15,f). The bodies of the vertebra are of a different shape in the three kinds, as may be seen from the figures. The two uppermost ver tebrae are peculiar. The first is called the atlas (Fig. 18) because it bears the head. It has very large surfaces (g) for a joint between it and processes of the occipital bone, and it is owing to this joint that the nodding movement of the head is possible. This vertebra has no body, but, instead, a small ring, separated from the large one (1) by a strong fibrous band which passes across between the two (see arrow lines in Fig.
18, k). The second vertebra is called the axis (Fig. 19), because its union with the first is such as to permit a turning movement between them, by which the head is turned to one side or another.:- This is effected by means of a process called the odontoid or tooth-like pro cess, Fig. 19, c, b, e, which springs upwards from the body of the axis and fits into the smaller of the two rings of the atlas. The odontoid process is retained in this position by means of a joint between the surface e (Fig. 18) of the axis and the surface d (Fig. 19) of the process, as well as by „c strong fibrous bands, d b which also prevent it turning too far, and par ticularly by the ligament already mentioned as passing across between the two rings of the atlas. This ligament, in the living state, inter venes between the process in front of it and the spinal marrow behind it, and if it were to break, the process would crush back whrds,destroy the spinal cord at this point, and so cause instant death. This occurs when the neck is broken.