Page:EB1911 - Volume 23.djvu/182

ANATOMY] perforates the right liver, receiving from it many hepatic revehent veins and also the big revehent vessel from the left lobe; next it receives the coronary vein and then enters the heart as inferior vena cava. The portal vein arises out of the coccygo-mesenteric (which comes out of the bifurcation of the caudal), collecting the blood from most abdominal viscera and from the thorax and breaks up in the right liver. The rest of the venous system is rather complicated. The big caudal vessel divides near the vent, receives an unpaired cloacal and a rectal vessel, and goes off to the right and left, each of which trunks receives an ischiadic and an inter-sacral vein and then divides into-the v. renalis advehens which breaks up in the kidney, and the abdominal vein. The latter are interesting; they run in the abdominal wall, receive the obturator and other pelvic veins, intervertebrals and intercostals, the crurals, and the epigastrics out of the body-wall. Then these two abdominal (Rathke's internal epigastrics) go to the liver, which they enter to either side of the gall bladder, collecting also blood from the stomach and from the vertebral column: Both break up in the liver. Consequently all the blood from “below the heart” passes through some portal system—renal or hepatic—except that which comes from the genital glands and ducts and from the fat body.

Tortoises.—The venous system much resembles that of the crocodiles, but many and wide anastomoses, especially on the inside of the carapace and plastron, exist between often distant vessels, so that one lucky injection may fill the whole system. There are three advehent renal veins which collect on the back of their kidney into one stem; they dissolve completely into a portal system, and leave the kidney on its ventral surface as one v. renalis revehens. The right and left then form the v. c. posterior which perforates the posterior margin of the right liver, then headwards of the liver takes up the hepatic and enters the heart. The three pairs of afferent renal veins are composed as follows. The externa collects from the shell and the abdominal muscles; the posterior collects along the rectum from the genital glands, the bladder, and from parts of other pelvic viscera; the anterior comes from the anterior part of the shell and runs backwards to the kidney, with frequent anastornoses with the other advehent renal veins. The abdominal arise, as in the crocodiles, with the external advehent renal from the lateral continuation of the bifurcated caudal, which takes up vessels from the pelvis, the shell and the crural. The abdominal itself takes up a femoral vein, vessels from the abdominal and pelvic muscles, and from the plastron, and then dives into the body-cavity, receives veins from the fore limbs, and enters the right lobe of the liver, there to break up. The hepatic portal collects from the intestinal tract, spleen and pancreas. Consequently in tortoises all the blood from below the heart passes through some portal system.

The most important peculiarity of the Lizards is the condition of the abdominal veins; they combine into a single stem (after having collected the blood from the fat body and from the ventral body-wall of the pelvic region) which dives into the body-cavity to join, embedded in the ventral hepatic ligament, the left branch of the portal vein. The chief characteristic of the abdominal is that it does not communicate directly with the caudal, and that it forms an unpaired stem. The renal portal system receives its blood from the tail, the hind limbs, the abdominal wall and the urino-genital organs, all the blood passing into a right and a left advehent vein. The suprarenal portal system drains from the abdominal wall and the suprarenal bodies, and issues into the revehent renals. These, with some intervertebrals and with hepatics, constitute the inferior vena cava.

The lymphatic vessels frequently accompany the big arteries of the trunk, either surrounding them with a meshwork or ensheathing them completely, especially in tortoises. The lymphatics from the head and neck combine with stems which accompany the veins of the fore limbs; they join the thoracic

ducts and these open into the brachio-cephalic veins, as they do in birds. The lymph from the tail flows into the ischiadic veins or into the advehent renal veins. Reptiles possess only a posterior pair of lymph-hearts; they are placed near the root of the tail against the ends of one of the transverse processes. In snakes they lie in a space protected by the ribs and transverse processes of the original sacral vertebrae. Lymph glands proper are not developed in reptiles, except in the shape of the so-called mesenteric gland of crocodiles.

The red corpuscles are invariably oval, and, since they still possess a nucleus, biconvex. Numerous measurements have been made by G. Gulliver (P.Z.S., 1845, pp. 93-102), their long and short axes range between 0.015-0.023 and 0.009-0.21 mm. respectively. That means to say they are very much larger than those of mammals, considerably larger than those of most birds, and in turn much smaller than those of amphibia.

Teeth..—All the groups of recent reptiles have teeth, except the tortoises, which have lost even embryonic traces of them. In the under jaw they are restricted to the dentary bones. In the upper they are almost universal in the maxilla and premaxilla, although the latter has lost them in most of the snakes. The pterygoids are toothed in most snakes and in a few lizards, e.g. Lacerta and Iguana. The palatines are toothed in Sphenodon and in some lizards.

Only the young of Sphenodon and the chameleons have a few small teeth on the vomer. The teeth themselves consist of dentine with a cap of enamel and with cementum around their base. In the crocodiles they are planted into separate alveoles in the maxilla, premaxilla and under jaw. In lizards they are either pleurodont, i.e. they stand in a series upon a longitudinal ridge which projects from the lingual side of the supporting bone, or they stand upon the upper rim of the bone, acrodont. In either case they are, when full grown, cemented on to the bone. Acrodont are amongst lizards only the Agamidae; the Tejidae are intermediate, almost acrodont. All the snakes and Sphenodon are acrodont. The latter is in so far peculiar as its broad-based, somewhat triangular teeth are much worn down in old specimens; originally there are several in the premaxilla, but the adults bite with the somewhat curved-down portions of the premaxillaries themselves, or with what remains of the anchylosed bases of the original teeth, which then, together with the bone, look like a pair of large chisel-shaped incisors. The lateral edges of the palatines of Sphenodon likewise carry teeth, those of the mandibles fit into a long slit-like space between the palatine and the maxillary teeth. This is a unique arrangement. Further, it is surprising that in this old, Rhynchocephalian type the supply of teeth has become exhausted, whilst in the other recent reptiles the supply is continuous and apparently inexhaustible. The new teeth lie on the lingual side of the old set, and long before the new tooth is finished part of the base of its older neighbour is absorbed, so that the pulp-cavity which persists in nearly all reptilian teeth becomes free. Ultimately the old tooth is pushed off and the new is cemented into its place. In the Crocodiles it has come to pass that several sets of teeth are lodged more or less into one another's bases. Where Crocodiles and alligators collect habitually the ground is sometimes found strewn with thousands of teeth, large and small, every creature shedding about seventy teeth many times during its long life.

Some or all teeth of various families of lizards and snakes have a more or less pronounced groove or furrow along their anterior convex curve. The usefulness of this furrow in facilitating the entering of saliva into the bitten wound is merely incidental, but this preformed feature has in many snakes been improved into a fearful weapon. In the Opisthoglypha a few of the most posterior teeth in the maxilla are enlarged, have deeper furrows, and lie in the vicinity of the poison ducts. In the Proteroglypha one or two of the most anterior maxillary