Thoracic Vertebrae.—There are twelve of these in the human spine and all are distinguished by having one or two smooth sur faces on each side of the body for articulation with the head of one or two ribs. The transverse processes have an articular sur face in front for the tubercle of a rib. The first thoracic vertebra is very like the seventh cervical; the twelfth has its lower articular processes shaped like those of a lumbar vertebra.
Lumbar Vertebrae.—These in man are five in number. They are the largest of the true verte brae, especially in the centrum. The fifth lumbar vertebra has its body much deeper in front than behind and its spine is less mas sive.
Sacrum.—The sacrum is com posed of five originally separate vertebrae fused into a single bone. It forms the upper and back wall of the pelvis, is trian gular in form, and possesses two surfaces, two borders, a base and an apex. The anterior or pelvic surface is concave, and is marked by four transverse lines, which indicate its original subdivision into five bones, and by four pairs of foramina, through which are transmitted the anterior sacral nerves. Its posterior surface is convex; in the middle line are four spines. On each side of these are two rows of tubercles, the in ner of which are the conjoined articular and mammillary processes, the outer the transverse processes of the originally distinct vertebrae. Between these rows four pairs of foramina transmit the posterior sacral nerves from the sacral canal, which extends through the bone from base to near the apex, and forms the lower end of the spinal canal. By its borders the sacrum is articulated with the haunch bones—by its base with the last lumbar vertebra, by its apex with the coccyx. The human sacrum is broader in proportion to its length than in other mammals ; this great breadth gives solidity to the lower part of the spine, and, conjoined with the size of the lateral articular surfaces, it permits a more perfect junction with the haunch-bones, and is correlated with the erect position. Owing to the need in woman for a wide pelvis, the sacrum is broader than in man. (For details see A. M. Paterson, "The Human Sacrum," Sci. Trans. R. Dublin Soc. vol. v. ser. 2.) Coccyx.—The coccyx consists of four or five vertebrae in the human spine though the last one is sometimes suppressed. It is
the rudimentary tail, but instead of projecting back, as in mam mals generally, is curved forward, an arrangement also found in the anthropoid apes and in Hoffmann's sloth. The vertebrae of which it is composed are small, and represent merely the bodies and transverse processes of the true vertebrae. There are no arches.
The human spine is more uniform in length in persons of the same race than might be supposed, variation in the height of adults being due chiefly to differences in the length of the lower limbs. The average length of the spine is 28 in.; its widest part is at the base of the sacrum, from which it tapers down to the tip of the coccyx. It diminishes also in breadth from the base of the sacrum upwards to widen again in the region of the neck.
i Behind and laterally it presents an irregular outline, but in front it is more uniformly rounded, owing to the convex form of the antero-lateral surfaces of the bodies of its vertebrae. In its general contour two series of curves may be seen, an antero posterior and a lateral. The antero-posterior is the more im port'ant. In the infant at birth the sacro-coccygeal part of the spine is concave forward, but the rest of the spine is almost straight. When the infant begins to sit up, a convexity forward in the region of the neck appears, and subsequently, as the child learns to walk, a convexity forward in the region of the loins. Hence in the adult spine a series of convexo-concave curves are found, which are alternate and mutually dependent, and are asso ciated with the erect attitude of man. A lateral curve, con vex to the right, opposite the third, fourth and fifth thoracic vertebrae, with compensatory curve convex to the left immediately above and below, is due apparently to the greater use of the right arm. In disease of the spine its natural curvatures are much in creased, and the deformity known as humpback is produced. As the spine forms the central part of the axial skeleton, it acts as a column to support not only the weight of the body, but of all that can be carried on the head, back and in the upper limbs : by its transverse and spinous processes it serves also to give attachment to numerous muscles, and the transverse processes of its thoracic vertebrae are also for articulation with the ribs.