'ervical ertebræ.

Dorsal

term Spine, applied to it. The transverse processes project outwards, one from each side of the arch. The articular processes project, two upwards and two downwards, and are for connecting adjacent vertebræ together. The Notches, situated on the upper and lower borders of the pedicles, form in the articulated spine the intervertebral foramina through which the nerves pass out of the spinal canal.

The vertebræ in each group have characters which specially distinguish them. In man and all mammals, with few exceptions, whatever be the length of the neck, the Cervical Vertebræ are seven in number. The exceptions are the three-toed sloth, which has nine, and Hoffmann's sloth and the manatee, in which there are only six. In many whales the seven cervicals are fused in the adult into a single bone. In man the body of a cervical vertebra is comparatively small, and its upper surface is transversely concave; the arch has long and obliquely sloping laminæ ; the ring is large and triangular; the spine is short, bifid, and horizontal; the transverse process consists of two bars of bone, the anterior springing from the side of the body, the posterior from the arch, and uniting externally to enclose a foramen, through which, as a rule, the vertebral artery passes; the articular processes are flat and oblique, and the upper pair of notches are deeper than the lower. The first, second, and seventh cervical vertebræ have characters which specially distinguish them. The first, or Atlas, has no body or spine: its ring is very large, and on each side of the ring is a thick mass of bone, the lateral mass, by which it articulates with the occipital bone above and the second vertebra below. The second vertebra, Axis, or Vertebra dentata, has its body surmounted by a thick tooth-like odontoid process, which is regarded as the body of the atlas displaced from its proper vertebra and fused with the axis. This process forms a pivot round which the atlas and head move in turning the head from one side to the other; the spine is large, thick, and deeply bifid. The seventh, called Vertebra prominens, is distinguished by its long prominent spine, which is not bifid, and by the small size of the foramen at the root of the transverse process. In the human spine the distinguishing character of all the cervical vertebræ is the foramen at the root of the transverse process, but amongst mammals this is not an invariable character, for in the cetacea the transverse process of the atlas is imperforate, and in the horse, ruminants, and many quadrumana, the seventh cervical vertebra has no foramen at the root of its transverse process.

The Dorsal Vertebræ, more appropriately called costal or vertebræ. thoracic, are twelve in number in the human spine; but amongst mammals they range from eleven in the armadillo to twenty-two in the Cape hyrax and Hoffmann's sloth. They are intermediate in size and position to the cervical and lumbar vertebræ, and are all distinguished by having one or two smooth surfaces on each side of the body for articulation with the head of one or two ribs. The arch is short and with imbricated lamina; the ring is nearly circular; the spine is oblique, elongated, and bayonet-shaped; the transverse processes are directed back and out, not bifid, and with an articular surface in front for the tubercle of a rib; and the articular processes are flat and nearly vertical. The first, twelfth, eleventh, tenth, and sometimes the ninth, dorsal vertebræ are distinguished from the rest. The first is in shape like the seventh cervical, but has no foramen at the root of the transverse process, and has two articular facets on each side of the body; the ninth has sometimes only one facet at the side of the body; the tenth, eleventh, and twelfth have invariably only a single facet on the side of the body, but the eleventh and twelfth have stunted transverse

processes, and the twelfth has its lower articular processes shaped like those of a lumbar vertebra.

The Lumbar Vertebræ in man are five in number, but Lumbar amongst mammals they range from two in the platypus to vertebra. eight in the hyrax or agouti. They are the lowest of the true vertebræ, and also the largest, especially in the body. The arch has short and deep lamina; the ring is triangular; the spine is massive and hatchet-shaped; the transverse processes are long and pointed; the articular are thick and strong, the superior pair concave, the inferior convex, and the inferior notches, as in the dorsal vertebræ, are deeper than the superior. In the lumbar vertebræ and in the lower dorsal an accessory process projects from the base of each transverse process, and a mammillary tubercle from each superior articular process. In man these are small and rudimentary; but in some mammals, as the kangaroo, armadillo, and scaly ant-eater, the mammillary tubercles are large, and in the baboon, dog, cat, and beaver, the accessory processes are well developed. The fifth lumbar vertebra has its body much thicker in front than behind its spine is less massive, and its lower articular processes are flat.

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The Sacrum is composed of five originally separate Sacrum. vertebræ fused into a single bone. In the bandicoot it consists of a single vertebra, whilst it has as many as eight in the armadillo. The relative size and completeness of the sacrum are associated with the development of the haunch bones and of the lower limbs. In whales, where the pelvic bones are rudimentary and there are no hind limbs, there is no sacrum. It forms the posterior wall of the pelvis, is triangular 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 tubercles or rudimentary spines, and on each side of these are two rows of tubercles, the inner of which are the conjoined articular and mammillary processes, the outer the transverse processes of the originally distinct vertebræ; in addition, four pairs of foramina are found which transmit the posterior sacral nerves from the sacral canal, which extends through the bone from base to apex, and forms the lower end of the spinal canal. By its borders the sacrum is articulated with the haunch-bonesby 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.

The Coccyx consists of only four vertebræ in the human Coccyx spine. It is the rudimentary tail, but instead of projecting back, as in mammals generally, is curved forwards, and is not visible externally, an arrangement which is also found in the anthropoid apes and in Hoffmann's sloth. In the spider monkeys as many as thirty-three vertebræ are found in the tail, and in the long-tailed pangolin the number reaches forty-six. Not only is the tail itself rudimentary in man, but the vertebræ of which it is composed are small, and represent merely the bodies of the true vertebræ. As there are no arches, the ring is not formed, and the spinal canal does not extend, therefore, beyond the apex of the sacrum. The first coccygeal vertebra, in addition to a body, possesses two processes or horns, which are jointed with two corresponding processes from the last sacral vertebra.