Page:The New International Encyclopædia 1st ed. v. 01.djvu/419

ALIMENTARY SYSTEM. wall of the pharynx behind the last gill-slit: the paired masses are called "accessory thj'roid glands." The median part is morphologically the most important. It is the only part found in Amphioxus and Cyclostonii. In these groups it exists as a groove in the ventral wall of the pharynx, called the hypobrancliial groove." A similar groove is found iu all tunicates (the "endosty le" ), and is glandular in function. In the lower true vertebrates, where the paired components first arise, they remain distinct; in mammals all components fuse.

Esophagus and Stojiacii. These parts of the alimentary tract constitute the fore gut in the more restricted sense. They are limited ante- riorly by the oral cavity; the limitation is a sharp one. however, onl}' in mammals, which possess a soft palate that curtains off the mouth from the respiratory passage. This soft palate makes its first appearance in the crocodiles, but withoTit the uvula. The posterior limit of the fore gut is not always easy to fix, since not all vertebrates have a specialized stomach with a pj'lorie valve. The opening of the bile duet may be taken as the lower limit. The post-pharyn- geal prosenteron is extremely short in Amphi- oxus and the lowest vertebrates, and is of rela- tively slight importance; it gains size and impor- tance as we ascend in vertebrate series. The digestive function is, in the higher groups, trans- ferred to a more anterior region of the enteron, and, coincidently, the entire alimentary tract, which is primitiveh' straight, undergoes a great increase in length and becomes strongly folded. A differentiation of the prosenteron into oesoph- agus and stomach is first indicated in selachians, and becomes pronounced in Amphibia. The two organs differ not onl' in their diameter, but also in the cliaractcr of the mucous membrane, which is smooth and forms a ciliated epithelium on the oesophagus and folded and nonciliated in the stomach. In birds the oesophagus is specialized, in that it is greatly enlarged at one point, form- ing the crop. The crop is best developed in gran- ivorous birds; in it grain is acted upon chemi- cally. Certain fish-eating birds have a reservoir (false crop) for excess of food. Insectivorous and frugivorous birds have no sign of a crop. The stomach, likewise, is very complex in birds. There is first a highly glandular chemically active provoitriciiliis, and, below, a mechanic- ally acting muscular stomach or gizzard. In mammals the stomach is the most distended and one of the most functional parts of the alimentary system. An anterior or cardiac portion can be distinguished from a posterior pyloric part. The stomach is larger and more coni])lex in herbivores than in carnivores. In the herbivores the cardiac and pyloric parts are each divided into two parts. The first is a large sac called the pouch or rumen. It communicates broadly with the second chamber, the reticulum, so called from its network of folds. Next comes the psdltrritim. whose walls are raised into high, thick-set folds, so that under most circumstances nothing but semi-fluid materials can pass between the folds. The last part is the abomq,sum, with highly vascular and glandular walls. This com- plex stomach seems to have arisen by natural selection as an adaptation to the peculiar habits possessed by the ruminants. They are all weak, defenseless mammals, and their herbivorous habits require that they shall feed in open fields where the danger of detection by the larger car- nivores is very great. The shorter the time they are e.posed in the open field the less will be the chance of their destruction. It has therefm-e been of advantage that they have become able to crop a large amount of grass rapidly without masticating it, the mastication being first done after the ruminant has retired from the field to the secluded forest. The food thus taken into the stomach fills the rumen and reticulum, and is mingled with and partly macerated by the saliva. By the action of the abdominal muscles anil diaphragm, as in hiccoughing, the food is returned to the mouth and is there masticated. Finally, divided and mixed with saliva, it passes down the oesophagus and is led b_v means of a special fold directly to the psalterium, through the leaves of which the finely triturated mass can pass. In the fourth part, or abomasum, true gastric digestion now occurs.

Inte.stin.l Region. The mesenteron is, in the higher vertel)ratcs. separated from the prosen- teron by a circular fold of the intestinal wall, the pyloric valve. The function of the tract is, on the one hand, to secure fluids that will finish the work of digesting the food which was begun in the stomach, and on the other to absorb the products of digestion. The variations in form of the mesenteroii are all to enable it to perform the processes to better advantage. Some of the vari- ations are readily visible to the naked eye, such as the foldings of the gut or out-pocketings from it ; others are microscopic, and due to foldings in the lining of the alimentary tract.

LiVEK AND Pancreas. The largest of these out-pocketings is the liver. It is phylogenetically an old organ, as it occurs in all the vertebrates, even in Amphioxus. The form of the liver is always closely adapted to that of the cavity in which it lies. In mj'xinoids it consists of two lobes, and this is probably the "ground-form" of the organ in all vertebrates. In. many cases it is further subdivided into (dog, weasel) six or seven lobes even. The right lobe is the larger, and in it the gall-bladder, when present, lies imbedded. The liver arises as an evagination of the epithelial lining of the ventral wall of the anterior portion of the intestine. In Amphioxus it is located just behind the gill region. The hepatic fundaments are soon transformed into glands made up of branching tubules. The network of tubules early differentiates into excretory and secretory parts. In amphibians and reptiles the tubular nature of the gland is easily recognizable, but in higher vertebrates (birds, mammals, man) the tubular structure is inconspicuous. Simultaneously with the development of the tubules a mesh work of blood vessels appears in the liver. In birds and mammals at the point at which the primary bile ducts open into the duodenum a small evagination is formed. This evagination elongates to form the bile duct. The gall-bladder is a reservoir for storing the gall. It develops as an evagination of the bile duct or from the hepatic ducts. The liver serves as a storehouse in Avhich the sugar not needed by the system for immediate consumption is stored up in the form of glycogen; it destroys the old red blood corpuscles and oxidizes nitrogenous materials into urea. Its function in digestion is less clearly understood. Fats, however, more easily pass through a membrane moistened with bile, and a greater proportion of fat passes unabsorbed through the intestine of a dog when the bile duet k stopped; hence bile probably aids in