The neural arches of the coccygeal vertebrae in man are always incomplete. The neurapophyses of the first piece terminate in articulating processes, which in many instances coalesce with the sacrum: the succeeding vertebrae are marked by a superficial groove.
The confluence of the five sacral vertebrae reduces the neural canal to a continuous bony tube, which is bent, and with the concavity directed forward. There are usually no more than three or four neural spines; the last sacral vertebra being incomplete posteriorly, like the coccygeal vertebrae. The prominent ridge, lying to the outer side of the posterior sacral foramina, and running parallel to the neural spines, indicates the situation of the coalesced di-apophyses and articulating processes. The greater part of the mass of bone external to this lateral ridge may be reduced to three or four pleurapophyses, or ribs, which, springing from the centra, incline backwards, and coalesce with the projecting diapo-physes. That these parts should be properly understood, it is necessary to examine the sacrum of a full-grown foetus.
In the lumbar, dorsal, and cervical regions the neural arch is complete in every vertebral segment. That portion of the neurapophysis which springs from the centrum is constricted, or notched above and below, and forms with the contiguous vertebrae, when in situ, the intervertebral foramen, or the opening for the passage of the spinal nerves. In the lumbar region the stunted pleurapophysis has coalesced with the root of the neurapophysis, and with the diapophysis, which, as in the lower dorsal region, has resolved itself into three heads: the epidiapo-physis, internally; the extremity of the diapophysis, united with the pleurapophysis, externally; and the met apophysis mesially. In very many Mammalia the metapophysis is prolonged downwards to the outer side of the superior articulating process of the vertebra below, which is thus wedged in between two bony spines : in man, the metapophysis commences about the ninth or tenth dorsal vertebra, and disappears about the third lumbar. It affords attachment to the tendons of the longissimus dorsi muscle. The neural spines are, in great measure, formed by the confluence of the neurapophyses ; but about the sixteenth or eighteenth year a thin plate of bone, developed upon their extremity, shuts up the otherwise open cancellous structure. Their direction is influenced by the amount of mobility in the different regions of the trunk. In the immovable dorsal region they are oblique, and imbricated; in the cervical and lumbar regions they are directed horizontally backwards. In the former of these regions the bifid spines indicate the union of the two neurapophyses : they are shorter than in the back, and connected together in the fresh subject by the elastic structure called the ligamentum nuchas. The length of the transverse processes of the atlas, and of the neural spine of the axis, is associated with the development of the small recti and obliqui capitis muscles, which are so intimately concerned in the movement of the head. These processes are particularly large in the lion and tiger; indeed, in all animals that use their jaws for seizing and lacerating their prey.
The neurapophyses (exoccipitals), and the neural spine (supra-occipital), of the first cranial vertebra, in man, offer a striking contrast to the narrow and compressed elements forming the neural arch of the atlas. But in the crocodile the spine of the atlas is nearly as broad and expanded as the occipital spine; and in the cod and the turtle the occipital spine preserves its compressed form, and is prolonged forwards, as a prominent ridge, between the two parietals.