Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/466

Rh 448 REPTILES [ANATOMY. hypapo Fio. 4. Lateral aspect of three thoracic vertebrae are of Crocodilus vulgaris (from nature). c,cnp on the anterior surface of centrum ; cp, capitula of ribs; neural spines. ^ rfb $" . tubercu i a O f piece (fig 4). From seven to nine of these ribs have ossified sternal ribs which serve to connect them with the sternum. The lumbar vertebrae have only long transverse processes which arise above the neuro-central suture. The sacral vertebrae support very stout transverse processes, each of which includes a rib element and becomes anchylosed to the centra only at a late period of life, as do the comparatively long and slender caudal transverse processes. Inferior processes, p/iyses, WHICH arvrrnnci rJprAnr1 nnp azygous, a< - from beneath the ribs ; u, uocinate processes ; vr, vertebral ribs. centrum of each of the more posterior cervical and anterior dorsal vertebrae. The sternum consists of an ossified rhomboidal plate, with the hinder side of which two pairs of costal cartilages articulate. A narrower piece extends backwards from this plate (connected with it by ligament), and ends in two diverging processes. With this the other sternal ribs articulate. A long slender bone, the interclavicle, or episternum, lies in a median groove of the sternal plate, and extends forwards beyond it. There is a so-called abdominal sternum made up of a series of seven or eight slender bones which lie in the aponeurosis of the external oblique muscle of the abdomen behind the true sternum and in front of the pelvis. It lies at a lower level than the sternum, and has no direct connexion with the vertebral column. In Hatteria, the only living member of the order Rhynchocephalia, as well as in the extinct species, the vertebral bodies have flat or concave surfaces in front and behind. There are 8 cervical, 14 dorsal, 3 lumbar, 2 sacral, and 36 caudal vertebrae. There is no persistent neuro- central suture in Hatteria, though there may be in some extinct forms of the order. Eudiments of the proatlas are sometimes present. The atlas consists but of three pieces, and the odontoid bone is anchylosed with the atlas, and is concave anteriorly. An autogenous hypapophysis is wedged into the inferior interspace between the centrum of the axis and the third vertebrae, and similar parts are developed thence backwards to beneath the seventh and eighth vertebrae. Hatteria has the faculty of reproducing its tail after mutilation. When the tail is broken the fracture takes place in the middle of one of its centra and not between two adjacent centra. This is owing to the fact that each caudal vertebra is divided into an anterior and a posterior part (fig. 5), and is weakest at this line of junction which passes behind the transverse process. The first rib is attached to the fourth vertebra. Those FIO. c. Vertical section of four (7th of the ninth and the following ^>C%S*Zrfi ffi Vertebrae attain the Sternum, middle of centrum. ( After Giinther.) The more posterior ribs become connected with the abdo- minal sternum. All the ribs are single at their proximal ends, a bifurcation being scarcely indicated even at the most anterior one. Processus uncinati are fully developed. The sternum is a rhomboidal, semi-cartilaginous plate, with a medianly situated long and slender episternum which unites with a pair of clavicles. An abdominal sternum is more fully developed than in any other living Reptile. It consists of about twenty-five transverse rods, each of which is composed of three pieces. These rods are con- nected and disconnected with the ribs in an alternate manner, every other rod being suspended from a pair of the true ribs. Lacertilia. In this extensive order of living Reptiles, j n ^ j the vertebrae are proccelous except in the Geckotidx and tiliai Uropeltidss, where they are biconcave. There may or may not be distinct cervical, lumbar, and sacral regions, and the number of vertebrae, generally considerable, may be very large. The distinctness of the vertebral regions depends on the development of the anterior and posterior limbs. When a cervical region can be distinguished, there are never more than nine such vertebrae. Only in rare instances are there distinct lumbar vertebrae. The sacral vertebrae are never more than two. The neural arch is always anchylosed to the centrum in adults. The atlas consists of three parts, and the axis may or may not have the odontoid bone anchylosed to it and convex in front. Ribs are attached to most of the cervical as well as to the dorsal vertebrae ; but the former are not expanded distally as in the Crocodile, and thus the latter do not support uncinate processes. The more anterior ribs show two proximal articular surfaces, but these never diverge into distinct capitular and tubercular processes as in the Croco- diles, nor, as in the latter, do any of the dorsal vertebrae develop double, superimposed transverse processes. The more anterior (usually the first three or four) dorsal ribs are connected with the sternum by sternal cartilages. Those behind may be similarly connected with the diverg- ing backward prolongations of the sternum, or may be directly connected with their fellows of the opposite side, as in the Chamaeleon, a median cartilage being joined on either side by the sternal cartilage continuous with an ossified rib. In Acontias and other limbless Lacertilians which have but a rudimentary sternum, or none, the corre- sponding ribs of the right and left sides are connected across the mid-ventral line as they are in Chamseleo. In Draco the more posterior ribs are greatly prolonged, and their distal cartilages are bent backwards. This is to enable them to support the wing-like membrane which extends from either side of the body to serve as a parachute. In those Lizards which have a lumbar region, transverse processes are there developed. The caudal vertebrae (except the most anterior and the small posterior ones) have chevron bones, which are not generally anchy- losed to the vertebrae which support them. In addition to the ordinary articular processes, there are in Iguana certain accessory articulating structures such as are commonly found in the vertebrae of Serpents; these will be described in the next section. The sternum consists of a rhomboidal semi-ossified or cartilaginous plate, which is sometimes continued back- wards into a pair of long diverging processes. There is generally an episternum, which is often T-shaped, but it may be absent, as in Chamaeleons. In the limbless Lizards the sternum is cartilaginous, and there is none whatever in some forms, as, e.g., in Amphisbsena and Typldine. In many Lizards, notably in the commonest English Lizard, the tail if broken off can be reproduced. In them the centra of the caudal vertebrae have a vertical median division similar to that already described in Hatteria. In Ophidians the number of vertebrae is generally very in hi- large, and may exceed four hundred. They are all proccel- ous, and the ball behind each is nearly hemispherical. There is no distinction of cervical, lumbar, or sacral vertebrae ; but all the vertebrae after the atlas may be considered as either dorsal or caudal. The mode of articulation between the vertebrae is more complex than in almost any Lizards.
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