Page:The New International Encyclopædia 1st ed. v. 01.djvu/420

ALIMENTARY SYSTEM. some way in the absorption of fats. The alka- linity of the bile also aids the pancreatic juice in overcoming the acidity of the gastric juice. Bile also aids in stimulating the action of the muscles of the intestine. Its absence leads to biliousness and even jaundice, and finally the bile acts as a preservative when deficient putre- faction of the contents of the alimentary canal results. The pancreas also arises as an evagination of the alimentary tract, but from the dorsal side of the duodenum, and usually opposite the origin of the liver. Except in cyclostomes and some telcosts, the pancreas is always present in vertebrates. Its size and form varies, and it is not infrequently lobate. In structure it is a racemose gland. Its secretion is either poured directly into the intestine (as in birds, crocodiles, Emydida>, and some mammals ), or as development proceeds its outlets move nearer and nearer the bile duct, and finally the secretions of the liver and pan- creas are poured into the intestine through a common duct. The pancreatic secretion is alka- line. Its role in digestion is very great. Its action on starch is like that of the saliva, only much more energetic. Through the agency of the ferment trypsin it eft'ects jiroteids, and by an- other ferment, steapsin, fats are split up into fatty acids and glycerine. The soap and glycer- ine are both soluble in water and hence are easily absorbed. Much of the fat, however, is emulsi- fied by the albumen, that is to say, it is broken up into fine drops, which are prevented from fusing by the presence of a coating of albumen. The fat in the emulsion is probably capable of absorption as it is.

The Pyloric Tract. The straight tube of Am- phioxus is chiefly an absorbing organ, the diges- tive secretions being poured into the cavity from the liver. In the earth-inhabiting Gymnophiona and AniphisbseniE and the elongated snakes, the alimentary tract is little convoluted, since here either the process of absorption is not very rapid, or the area of the mid gut is, even when straight, considerable, relative to the total volume of the body (snakes). The mesenteron is also straight in Petromj-zon and some of the sharks which lead an active carnivorous life, 1)ut the shortness is fully compensated for by an extensive folding of the inner absorbing surface through the forma- tion of the so-called spiral fold, or spiral valve. The method of origin of the fold is seen in Petro- myzon, where it is first represented by a strong, spirally twisted artery lying on the internal wall. This gradually sinks deeper and deeper toward the lumen of the gut, carrying tlic wall of the gut before it. As a result of this process we find a long, spirally twisted fold projecting far inward from the wall of the gut. The fold itself is richly vascular, from the ramifications of the small blood-vessels from the artery. Such a spiral valve is found best developed in Sela- chians, but it exists also in (ianoids. Teleosts lack such a spiral valve, but the absorbing sur- face is increased by another means — namely, by out - pocketings, so - called piiloiic appendages. That these are primarily not glands, but have an absorbing function, is indicated by two facts: (1) They are sometimes found slutTed with food, and (2) their presence seems to be correlated with the absence of the spiral valve and rire versa, even in closely allied species having similar habits; and therefore it is probable that they fulfill the same office in the economy of the organism. Thus, Polypterus possesses a well-developed spiral fold, but only a few pyloric appendages, while Lepi- dosteus, which is provided with only a slight fold, is superabundantly provided with pyloric appendages. All of this evidence is not quite satisfactory, and it seems probable that in some cases the pyloric appendages are indeed glandular — as, for instance, when several open into the mesenteron by a common duct. Histological studies are needed to settle this question.

The Mesenteron. — From the Amphibia on, with exceptions, the mesenteron becomes more and more convoluted externally, and at the same time the absorbing surface is increased by folds. I'hus, in the frog, the anterior part of the mesenteron is covered by a fine network of folds. Further posteriorly' thesc arrange themselves into structures like the semi-lunar valves of the heart, opening backward. Similar contrivances for in- creasing the internal absorbing surface are found also in reptiles and birds. In birds and mam- mals, when the longitudinal folds of the mesen- teron are poorly developed, we find finger-like processes — riUi — produced into the lumen of the gut. Into these folds of the mucous epithelium are continued the connective tissue of the sub- inucosa. together with blood vessels, lymph ves- sels, and nerves. Food in solution is taken up by the epithelial cells just as an amceba takes it up by tlirowing out pscudopodia. A large share of the absorptive process is probablv to be assigned to the lymjih cells, which wander about in the submucosa and even make their way through the mucosa into the lumen of the gut.

Mctenteron. — Like the other parts of the ali- mentary tract, this becomes differentiated from the common entercn only in the higher verte- brates. In the higher fishes it is indicated by an enlargement of the intestine. This enlargement is directly continuous posteriorly with the cloaca, into which also the urogenital ducts open. In Amphibia and reptiles the ventral wall of the hinder part of the mctenteron' is enlarged to form a (functional) urinary bladder. In Amniota the mctenteron is separated from the mesenteron by an ileo-ciccal valve (q.v. ). In nearly all vertebrates the metentcron — in contradistinction to mesenteron — has a straight course, hence it is often called rectum. In many mammals, as in man, it is greatly elongated, forming a colon ascendens, transversus, and deseendens. A blind pocket etecum is often formed in connection with the mctenteron. This is a mere swelling in the wall in reptiles, but attains an enormous development in many birds, in which group it is usually paired. In mammals it is never so long as in birds, but is variable in extent. Thus, in herbivores it may even be as long as the body of the animal possessing it, and in some rodents it contains a spiral valve. In carnivores, on the contrary, it is poorly dcAeloped. It would stem to be somewhat compensatory with relation to the rest of the metentcron. for it is much better developed in the horse and allies which have a simple stomach than in the ruminants with a complicated one. Among certain mammals (e.g. man) the distal part of the coccum is greatly reduced, forming the vermiform process. In man the c^cum is at first of nearly uniform character — the vermiform process arises by a degeneration of its distal end — a process which occurs relatively late. This indicates that in man the ctecuin was quite recently of relatively greater importance, and indicates further that man's