The vertebral column of Ophidians is, according to most authorities, divisible into three sections. - A cervical region containing only an atlas and axis: a second region containing vertebrae very numerous and corresponding to the posterior cervical, the dorsal lumbar and sacral regions in other Reptiles, and bearing freely moveable ribs, important organs of locomotion in these animals: and a third or caudal region. The last named is characterized by the anchylosis of the ribs to the centra, and the presence of bifid descending processes, which protect the caudal artery and replace the chevron bones found in all other Reptilia. The number of caudal vertebrae varies very much.
These vertebrae of a constricting Serpent are selected from the second section of the column. Each one has the following characters. The centrum is short and prismatic. Its anterior surface forms a deep cup with thin prominent edges and the vertebrae is therefore pro-coelous; its posterior surface has a ball peculiarly prominent. Both ball and cup are placed obliquely to the axis of the centrum. The inferior surface has a low ridge terminating in a knob posteriorly and representing the inferior spine (hypapophysis) seen in the anterior trunk vertebrae of the Python itself and in all the vertebrae of the common Ringed Snake and of venomous serpents. The neural canal is surrounded by a neural arch which is anchy-losed to the centrum as it is in Lacertilia and in Chelonia with few exceptions. The neural spine is low. At its base anteriorly, there stands above the neural canal a bony wedge, the zygosphene, with articular surfaces looking obliquely outwards and downwards. In a similar position but posterior to the spine is a deep cavity, the zygantrum, with articular surfaces looking obliquely inwards and upwards. The wedge and cavity fit the one into the other in contiguous vertebrae.
Similar structures are found in the Iguana among Lizards, and the Edentata with the exception of the Sloths among Mammalia. The articulating processes are remarkably large and flat The anterior pair (prae-zygapophyses), placed externally to the zygosphene, have their surfaces disposed typically, i. e. looking obliquely inwards and upwards, relations reversed in the posterior pair (post-zygapophyses). A low ridge connects the prae- to the post-zygapophysis of the same side.
The size of the articular surfaces, their disposition, the depth of the cup and of the zygantrum, and prominence of the ball and zygosphene permit great freedom of motion and at the same time prevent dislocation. The fact that the cup and zygosphene are both anterior, and the ball and zygantrum posterior, constitutes a further safeguard in the same direction.
Below the prae-zygapophysis lies the diapophysis or articulating surface for the rib. Its upper portion is convex in every direction, while the lower portion is concave from above downwards but convex from before backwards. This lower portion in some Snakes, e.g. Rattlesnake, is much prolonged ventrally.
The atlas resembles the corresponding vertebra in Lacertilia and Chelonia. It consists of three pieces: one inferior, prolonged ventrally into a spine; and two, one on either side, forming the neural arches. There is no neural spine. The odontoid process is united by anchylosis to the centrum of the axis. It carries an inferior spine united to it, at least in young specimens, by a suture and representing perhaps a sub-vertebral wedge-bone such as exists in many Lizards between the centra of two adjoining vertebrae. There are no ribs to the atlas and axis, but in a specimen of Python (sp. ?) in the Oxford Museum cartilaginous representatives of these structures exist The anterior caudal vertebrae, to the number sometimes of ten but never more, have ribs apparently forked at their vertebral end. The ventral division of the fork is perhaps an outgrowth from the centrum; i. e. represents the lower transverse process (= parapophysis). The lymphatic heart is lodged in the space enclosed by the fork.
Vertebral column.Owen and Bell, Reptilia of the London Clay, Part iii. Palaeontographical Society, 1850; De Rochebrune, Journal de l'Anatomie, etc. (Robin), 17, 1881.
Skull of Common Snake.W. K. Parker, Ph. Tr. 169, 1878.