Page:EB1911 - Volume 23.djvu/170

ANATOMY] Stegocephali and most Anura form the centre), therefore the typical batrachian vertebrae are notocentrous. If the remaining three pairs of constituent elements of each vertebra (the neural arch, the centrum and the intercentra) remain separate, the vertebrae are called temnospondylous (, I cut,, a vertebra). If the neural arches and the Centra are suturally united, or are fused with each other, the vertebrae are called stereospondylous (, solid). In many fossil reptiles most or many of the vertebrae are temnospondylous; in most of the recent Amniota they are consolidated, but the atlas or first vertebra remains usually in a relatively primitive condition, and is temnospondylous but for the usual modification that its centrum becomes attached to that of the second vertebra and forms its odontoid process. The composition of gastrocentrous vertebrae is best illustrated by the first and second cervical vertebrae of crocodiles, whence by reduction and fusion the structure of every other vertebra can be explained. We have only to add that the ribs are genetically derived from lateral outgrowths of the basiventral elements, whilst the chevron bones are mere ventral outgrowths from the same basal cartilages. The most primitive vertebral column is that of the Geckos. The vertebra consists chiefly of a large neural arch which rests broadly upon the centrum; this is a tube, more or less calcified and ossified, with a narrow waist in the middle, widening head- and tailwards. The tube is hollow, the chorda dorsalis passing through the whole column, and there are no proper joints between the centra, which are amphicoelous. Between the centra lies a separate element, the so-called intercentrum, which is ring-shaped and acts as an interarticular pad instead of a joint. The first of these rings forms the ventral half of the atlas ring; the second is attached to the cranial surface of the second centrum, and produces, like some of the next following ones, a vertical median blade of bone, a true hypapophysis. Such intercentra exist throughout the length of the vertebral column; in the tail they are enlarged and carry a pair of chevrons, which are cartilaginous and have the tendency of fusing by superficial

ossification on to the caudal ends of the centrum next in front, to which they do not belong genetically. Exactly in the middle of each vertebra the thin shell of the centrum forms a cartilaginous septum, of what is often wrongly called chordal cartilage. When this septum is complete, and this seems to be the normal condition in the tail, the chorda is here rent asunder, otherwise it is only constricted. This septum is but slightly invaded by ossification, and consists of large cells which retain the appearance of young or embryonic cartilage. It coincides exactly with the line of transverse division of most of the caudal vertebrae into an anterior and a posterior half, the division gradually extending right through the bone of the neural arch. The same kind of division, and from the same causes, exists in Sphenodon and in many lizards, in fact in all those reptiles which can reproduce their broken-off tail. It is from the septal cartilage that the regeneration starts (fig. 26).

Sphenodon also has biconcave vertebrae owing to the persistence of the chorda dorsalis in the intervertebral region; otherwise the vertebrae are solid. Intercentra occur from the atlas regularly into the tail, where they carry chevron bones. The atlas-ring (fig. 25, 9) is composed of the first intercentrum and a pair of neural arches which remain quite separate and carry on the dorsal side a pair of ossicles, the disconnected supra dorsal elements of the atlas, erroneously supposed to be the remnants of the “proatlas.”

Crocodiles.—Remnants of the chorda persist in the middle of the centra, which, in recent species, are mostly procoelous, and with a convex knob behind, but the first caudal is strongly biconvex. Cartilaginous intercentral rings, pads or menisci, occur throughout the column; in the tail they carry chevrons. For the instructive detail of the composition of the first and second cervical vertebrae see fig. 25, 7 and 8. Some of the posterior neck and anterior thoracic vertebrae have an unpaired hypapophysis arising from the centrum. The vertebrae have the usual processes, viz. spinous process, a pair of anterior and posterior zygapophyses arising from the neural arch, diapophyses likewise from this arch for the articulation with the tubercular portion of the rib; short parapophyses from the centra for the capitular ends of the ribs; the transverse processes of the 12th vertebra, and following, carry the whole rib, and are like the processes of the lumbar vertebrae diapapophyses; the so-called transverse processes of the tail are mainly the anchylosed or fused ribs themselves.

Chelonians.—The vertebrae are sometimes in the various regions of the same column opistho-pro- or amphicoelous, or even biconvex. Intercentra occur regularly on the first two or three cervicals, and on the tail as paired or unpaired nodules, or as chevrons, which articulate mostly with the previous centra and occasionally fuse with them. Intercentral, fibrocartilaginous disks occur regularly, mostly in the shape of rings; the first is the transverse ligament of the atlas-ring. In the Trionychidae (fig. 25, 10), but also in some other tortoises, the various pieces of the atlas do not anchylose, and the first centrum remains also movably attached to the second, although it sometimes carries,