Page:EB1911 - Volume 23.djvu/184

ANATOMY] more or less spindle-shaped, never transversely placed. The walls of the stomach are thrown into longitudinal folds which contain the specific gastric glands, whilst glands are absent in the oesophagus, excepting scattered and very simple slime glands. The circular muscular fibres of the stomach are much stronger than the longitudinal fibres. The end of the stomach is generally marked by a pyloric valve. The walls of the mid gut are said to be devoid of glands. The end gut, marked by a circular valve, is considerably wider and there is a caecum, mostly left-sided, largest in leaf-eating lizards, rarely absent, as, for instance, in Anguis. The absorbent portion of the rectum is always strongly marked off from the cloaca by a circular fold or sphincter, which projects into the widened coprodaeum of the cloaca. In those lizards which, like Varanus, have no urinary bladder, there are two successive sphincters, marking off two chambers, one, the upper or innermost, for the reception of the faeces, the lower for that of the urine. In adult crocodiles the stomach is transformed into a gizzard; it is more or less oval, with a wide fundus and with two opposite apo-neurotic or tendinous disks whence radiate the muscular fibres. The muscular walls remain, however, comparatively thin, like those of birds of prey. There is a distinct pyloric stomach and then follows the pylorus. The inner lining of the stomach is velvet-like with numerous gastric glands which form groups with net-like interstices. There is a distinct duodenal loop which contains the pancreas. The more convoluted mid gut is lined with net-like meshes which farther back assume a longitudinal zigzag arrangement; towards the end gut the walls become quite smooth, but in the end gut the walls again show a very narrow-meshed structure. None of these folds of the mid and hind gut is said to contain digestive glands; they seem to be entirely absorbent. The oesophagus of most tortoises shows longitudinal folds with very numerous mucous glands. In the Chelonidae the pharynx and adjoining part of the gullet are covered with little tubercles upon each of which opens a small gland. Farther down they give way to large, more or less conical papillae, which assume a considerable size, point backwards, and are covered with a somewhat horny epithelium. Similar conical, horny papillae exist also in Sphargis, in which the oesophagus, moreover, makes a long loop half round the stomach before passing into it, an absolutely unique feature. The transition into the stomach is quite gradual. The latter is strongly muscular, partly transversely placed, and possesses often a very distinct pyloric stomach. In Chelone conical papillae extend into the cardiac portion. In the majority of tortoises the inner lining shows longitudinal folds with numerous small glands, mucous and gastric, but their distribution differs much in the various families and even genera. The lining of the mid gut shows either longitudinal folds or a network, without glands, except in some cases, Lieberkühn crypts, e.g. in Trionyx, not in Testudo and Chelone. The hind gut begins suddenly, but there is no caecum; its inner walls contain numerous glands in Testudo, Emys, not in Chelys, Trionyx, Cinosternum.

In the snakes the oesophagus is very thin-walled and passes imperceptibly into the stomach, which continues in a longitudinal direction, scarcely wider in the middle. Its muscular coating is surprisingly weak. There is a small pyloric portion. Mucous and especially long-bodied gastric glands are numerous. The wall of the mid gut carries numerous papillae variably arranged, velvet-like, or densely crowded little blades supported by longitudinal or by meshy folds. The hind gut is short, often constricted into several successive chambers, mostly smooth inside; there is a short, rather wide caecum which seems best developed in Viperidae; sometimes absent. The total length of the snakes' gut is always short, there being only short folds possible or necessary in the body cavity, which itself is of extraordinary length. Yet, while in Typhlops the gut is almost straight, it forms numerous convolutions in Tortrix.

Whilst in all other reptiles the gut, at least stomach, liver and mid gut, are suspended by the mesentery from the vertebral column and hang free into the body cavity, in some snakes, especially often described in Boa and Python, the body cavity

is cut up into numerous spaces, by peritoneal folds which connect neighbouring twists of the canal into bundles and attach them to the ventral surface of the body-wall. Probably the gut is thereby secured against dislocations in adaptation to the peculiar twisting contortions of the body, especially in the act of climbing. The mesentery of reptiles is remarkable for the possession of smooth, non-striated, muscular fibres. In most lizards, not in other orders, the peritoneum so far as it covers the abdominal cavity shows a deep black pigmentation; this pigment is situated in the connective tissue, not in the epithelial layer; it stops suddenly towards the thorax. In some lizards, e.g. in Anguis, the black pigment extends, more or less scattered, upon the mesentery and thence upon the intestines. The same pigment colours the pharynx with its recesses entirely black in many lizards. There is no compensating correlation between this internal pigment and that in the outer skin.

The Liver of lizards is more or less bilobed; more so in crocodiles; while in tortoises the broad right and left lobes are connected by a narrow isthmus.. In the snakes it is much elongated and extends from the heart backwards along the right side of the oesophagus, closely connected in its long course with numerous short branches into, or from, the inferior vena cava and the portal vein. A gall bladder is always present. The ducts into and from the cyst sometimes form a complicated network, for instance in Varanus (F. E. Beddard); the bile is carried by one or more ducts into the duodenal portion of the mid gut. The microscopic structure of the reptilian liver has been compared with that of monotremes by M. Fürbringer.

The Pancreas is a compact body attached to the duodenal region, which surrounds it by a loop in the crocodiles, as is the case in birds and mammals.

The Cloaca of the reptiles shows a great advance upon the simple batrachian arrangement. It is no longer one common chamber, but consists of three successive chambers with the further tendency of separating the temporary retention and the passage of the faecal, urinary and genital products from each other. The arrangement is simplest and most typical in the lizards. There is first the proctodaeum or vestibulum of the cloaca, epiblastic in origin. Its outer boundary is formed by the cloacal lips, covered so far by the usual scaly integument. Just within this chamber arise the paired copulatory organs, and, when they are present, as in Sphenodon and snakes, the two anal glands. Secondly, the urodaeum, middle or urinogenital chamber, hypoblastic in origin. It is separated from the proctodaeum by a more or less circular fold which is provided with sphincter muscles, which form the true vent, and this is always round; whilst the outermost opening in lizards and snakes is a transverse slit. Farther inwards, headwards, the urodaeum is shut off by another circular fold, generally very well marked, especially in its dorsal half, which is higher and thicker. Into the dorsal, and innermost, recess of this urodaeum open the genital and urinary ducts; on the ventral side arises the urinary bladder. The whole chamber is always empty, being only a passage room, and in the female the copulatory chamber. The urine is of course collected in the bladder; when this is absent the fluid is pressed into the third chamber, the coprodaeum, which is often subdivided into two, or even three, successive rooms by circular folds. This coprodaeum serves for the temporary storage of the faeces, eventually mixed with the urine. Micturition and defecation are in most lizards two successive separate acts.

The snake's arrangement is a side-departure of that prevailing in lizards. The urodaeum is transformed into a dorsal recess into which open above the oviducts, while the ureters open below, in the caudal corner. A horizontal fold imperfectly shuts off the wide urino-genital chamber or recess from the ventral half of the original urodaeum. The coprodaeum is marked above and below by strong sphincters. There is no urinary bladder.

In crocodiles the protodaeum is rather shallow, but long; from its ventral wall arises the unpaired copulatory organ, the basal investing membranes of which continue into the ventral