In the generality of living reptiles (as, for example, in the Crocodile) the bodies of the vertebræ are concave in front, and convex behind; the bones of the spine being united by ball-and-socket joints; but, in most fossil reptiles, both faces are either flat, or more or less concave. In mammalian quadrupeds, the annular part is anchylosed to the vertebral centre; but in reptiles, it is united by suture, although, in old subjects, the connecting line is often obliterated. By reference to [Lign. 206], and its description, the form, arrangement, and connexion of the different vertebral elements, in certain fossil reptiles, may be easily understood. The bones in the vertebral column of the same animal are considerably modified in the several regions of the neck (cervical vertebræ), back (dorsal and lumbar), and tail (caudal). The cervical are generally of the most complicated structure; and the caudal, the most simple.
From this exposition, the reader will perceive that every vertebra consists of the following essential parts: first, the body, or centrum; and secondly, the annular part, or neural arch, so named, because it protects the nervous chord; while a caudal vertebra has, in addition, the chevron-bone, called also the hæmal arch, from its affording a passage to the large blood-vessels. The bodies of the vertebræ are in general solid, and consist of the ordinary osseous structure; but in certain fossil vertebræ the centre of the bone is filled with calcareous spar, indicating an irregular medullary cavity, as in the caudal vertebræ of the Ox.[584]
[584] See Petrifactions, p. 166, note.
The Sacrum, which may be termed the key-stone of the pelvic arch, is formed in existing reptiles by the union of two vertebræ; but in the Iguanodon and the Hylæosaurus the sacrum is composed of six anchylosed vertebræ; in the Megalosaurus probably of but five.[585]
[585] Report, Brit. Assoc. 1841, p. 105, and p. 130.
From the sides of the two anchylosed vertebræ which form the sacrum, strong, short, rib-like processes are given off in those Saurians which occasionally walk on dry land, and these constitute a firm support to the hinder extremities.
In the Crocodiles, the four or five vertebræ preceding the sacrum have no ribs attached to them, and are termed lumbar; in the Lizards, there are but two lumbar vertebræ. A peculiar modification exists in the first caudal vertebra of the adult Gavial and Crocodile; the centrum is convex both in front and behind, as was first demonstrated by me in 1836. See Wond. p. 419, and Petrif. p. 167. The last of the anchylosed vertebræ forming the sacrum is concave posteriorly; hence the necessity of an anterior ball in the first joint of the tail. (See [Lign. 217], p. 676, illustrative of Crocodilus Hastingsiæ.) The last cervical vertebra in the Turtles and Tortoises has a similar construction. This mechanism confers freedom of motion without risk of dislocation.
Ribs.—The Ribs, which are regarded as appendages to the vertebræ, (homologues of the pleurapophyses,) are generally slender and round in the Lizards, and articulate with the spinal column by a single head, supported on a short convex process or tubercle. In Crocodiles only of all existing Reptiles, but in several extinct genera, the proximal end of the rib forms a double articulation, by a distinct head and a tubercle, with the vertebræ in the cervical and anterior dorsal region of the spinal column; in the posterior dorsal region the ribs are attached to the elongated transverse processes of the vertebræ.
As this double articulation of the ribs is invariably associated in existing reptiles with a heart having double ventricles, while the lacertian single-headed ribs are in like manner connected with a heart having but one ventricle, the student will perceive the important physiological inferences that spring from the discovery of a mere fragment of a rib, when interpreted by the profound anatomist.[586] In some fossil reptiles the ribs are flat and very broad; as, for example, in the Hylæosaurus.[587]
[586] See Brit. Assoc. Report, 1841, and Memoirs, Palæont. Soc.