Page:EB1911 - Volume 23.djvu/172

ANATOMY] capitular and tubercular processes, which, articulating with the bodies and with dorsolateral processes of the neural 30.—Lateral aspect of Three Thoracic Vertebrae of Crocodilus vulgaris (after Mivart). c, cup on the anterior surface of Centrum; cp, capitula of ribs; ns, neural spines; t, tubercula of ribs; u, uncinate processes; vr, dorsal or vertebral portions of the ribs; rc, ventral or sternal cartilaginous portions of ribs. arches of their vertebrae, enclose typical transverse canals. In the posterior thoracic region the ribs are attached entirely to transverse processes of the neural arches, both capitular and tubercular portions having left the bodies or centra; the same arrangement prevails in the tail, but the ribs are very short and soon fuse with the processes. The two sacral ribs are very thick, articulating with the centra and the bases of their neupal arch, and they even form part of the intervertebral joint! In Sphenodon the first three ribs are represented by bands of connective tissue only, with similar attachments as in crocodiles. The other cervical ribs are osseous; their short capitula retain their partly intercentral attachment, while the tubercular are carried by low processes of the centra. In the thorax both capitulum and tuberculum merge into one facet, which is gradually shifting farther tail wards and upwards until the attachment reaches them, and then lies upon the neuro-central suture. The first caudal vertebrae also possess ribs, very short and soon fusing with the diapophyses of the neural arches. In the cervical region of the Chelonia the ribs seem to be absent. In the thorax they retain their primitive inter central position throughout life, assuming (except the first pair, which remains short and least modified) an absolutely intervertebral position. From the lumbar or presacral region backwards the capitula are gradually shifting upon short processes of the centra, until in the tail the vestigial ribs are carried by the diapophyses of the neural arches. In Sphargis (fig. 31) all the ribs are free; in 31.—Three Vertebrae of Sphargis coriacea. c, vertebral centra; n, neural arches; r, ribs. the other Chelonians the ribs, generally in the recent species, flatten and become surrounded by the growing membrane bone of the dorsal plates, and the cartilage of the ribs (except the capitular and neck portion of the rib, which cannot be got at by the dermal bones) undergoes a process of calcification. Ultimately this is resorbed and its place is taken by the dermal bone, which forms, so to speak, a cast of the rib. Several of the short presacral ribs, and of course the postsacrals, are not drawn into these enormous changes, although the carapace covers, and indirectly affects, them.

Certain changes initiated in Sphenodon are more marked in the ribs of the Lacertilia; cervical ribs are often long in the lower neck. In the trunk the capitular portions are often much reduced, and in these cases the ribs are suspended mainly by their tubercular portions, usually from the diapophyses of the neural arches near the anterior end.

In the snakes all the vertebrae, from the second cervical to the tail, carry ribs. These are very movable, articulating with a rather large, more or less vertically placed facet, which is borne by the parapophysis or transverse process; sometimes the

rib retains traces of the original division into a capitular and tubercular portion. The ribs of the snakes, although long, consist only of their dorsal portions. In snake-shaped lizards, e.g. Pseudopus, rather long ribs begin with the fourth vertebra.

Uncinate processes are developed only in Sphenodon and in the Crocodilia. They are not homologous structures, arising in the former from the posterior margin of the middle of the dorsal portions of the ribs, overlapping the shaft of the next following rib; in the crocodiles they arise out of the middle portion of the ribs, remaining cartilaginous, whilst the middle portion coössifies with the dorsal. Only in Sphenodon and Crocodiles the thoracic ribs consist of three successive pieces; in the Lacertilia they consist only of the dorsal and the ventral or costosternal. The latter remain cartilaginous, or they calcify, but they never ossify.

The sternum and further modifications of the ribs of the trunk.—The sternum of most reptiles consists (1) of an anterior portion (presternum, Parker; prosternum, Fürbringer; mesosternum of Gegenbaur), which is generally broad, more or less rhomboid and carries the shoulder-girdle, and on its posterior sides several pairs of ribs; (2) of posterior portion (mesosternum and xiphisternum of Parker; xiphisternum of Fürbringer; metasternum of Gegenbaur), which is narrow, sometimes metameric, carries several pairs of ribs, and generally divides into a right and left xiphoidal half, each of which is continued into one or more ribs. These ribs tend to lose their connexion, and in these cases the sternum ends in two typical xiphoid processes. The distinction between pre- and metasternum is arbitrary. In Sphenodon the broad sternal plate carries only three pairs of ribs, the 8th to 10th, and there is no xiphisternum. The other ribs of the trunk are long and compound, but they remain free and do not approach the mid-line. From the posterior edge of the sternum to the pelvis extends the complicated parasternum, embedded in the abdominal wall; it is composed of about two dozen sets of abdominal ribs, each set containing a right and a left and a median chevron-shaped piece. In the Crocodilia the presternum carries only two or one pair of ribs, always that of the 10th vertebra. The narrow, more or less metameric metasternum carries seven or eight ribs, the last one to three being xiphoidal. The post-thoracic ribs gradually decrease in length; about three presacral vertebrae have no ribs, and so are typically lumbar. The sacral ribs are generally the 25th and 26th in Crocodilus and Alligator; sometimes the 24th and 25th in Gavialis. The parasternum consists of only seven or eight transverse sets, each composed of two right and two left narrow splint-bones. All these parasternal elements belong to the category of dermal bones, together with those of the plastron of tortoises, inherited from Stegocephalian conditions.

The Lacertilia present an almost endless variety. The presternum is rhomboid and broad; it carries from three to six pairs of ribs, mostly four or five; the first thoracic rib is that of the 9th vertebra, the only exceptions being the chameleons with only five cervical vertebrae, and Varanus, which has usually nine cervicals like the crocodiles. The last cervical rib in these long-necked lizards is very long and has all the appearance of having but recently severed its connexion with the sternum. The presternum of Lacertilia sometimes has a window, e.g. some species of Lacerta, Phrynosoma, Iguana, or a pair of windows, e.g. Agama, Liolepis, Goniocephalus. The xiphisternum carries a variable number of ribs; it is either scarcely distinguished from the anterior plate, or it is long, and in these cases either double, e.g. Iguana, Gerrhonotus, Varanus, Zonurus, Agama, Cyclodus, Lacerta; or single, e.g. Zonosaurus. The poststernal ribs shorten gradually in the majority of the Lacertae, and there is sometimes a ribless lumbar vertebra, e.g. in Iguana; in many Lacertilia, however, the ventral cartilaginous halves of the ribs are connected with those of the other side, either by ligaments, or they join together, forming complete hoops of thin cartilages. Such ribs occur in all Geckones and Chameleons, but also in many Iguanidae, Scincidae, and even in the Anelytropidae; their numbers vary much, from 27 in the Scincoid Acontias meleagris, 7-10 in Polychrus, 8 in Chamaeleo