The Mouth.—The beginning of the alimentary tract is enlarged to form an oral or buccal cavity, provided with teeth and glands. The glands are modified skin glands, as would be expected from their ectodermal origin. The glands de velop by a depression of the epidermis, and come to lie imbedded deeply in the eutis of mesenehymatons origin. The function of the glands is to keep the mouth moist, consequently they arc found only in land vertebrates. The poison glands of serpents are modified oral glands. Salivary glands find their highest de velopment in mammals. They are probably im mensely developed skin glands or groups of such. They secrete a thick, glairy fluid, whose chief function is to moisten the food and thus to assist in its mastication and deglutition. On this account these glands are most highly developed in the Herbivo•a and are absent in Cetacea. Saliva also acts upon starchy food, converting it into sugar.
The tongue is a mass of intertwined muscles, having various functions, as of tasting, grasp ing, touching, and speaking. In fishes it is little developed, being represented by a thicken ing of the mucosa covering the ventral part of the hyoid bone. In Amphibia and reptiles it shows a great advance in size and complexity, being capable of extrusion to a great extent (especially in lizards), both through the elon gating action of its intrinsic muscles and the forward movement of the base of the hyoid bone.
The thymus gland arises in fishes by the budding off of epithelial masses from the ante rior four or five gill pockets; it is thus of multi plex origin. Usually these independently aris ing masses fuse into a pair of spindle-shaped bodies, but in the Gymnophiona the components persist as distinct bodies. In the land verte brates, with fewer gill-slits, the points of origin are reduced in number. Into the paired masses connective tissue and blood vessels grow, eventu ally constituting the greater part of the organs. The function of the thymus is still unknown. It attains its largest size in reptiles and birds. In man it reaches its maximum development in the second year and then gradually degen erates.
The thyroid gland arises directly from the ali mentary tract. It has a double origin. First, it arises as an unpaired pocket of the ventral wall of the pharynx behind the last gill-slit: the paired masses are called "accessory thyroid glands." The median part is morphologically the most important. It is the only part found in Amphioxus and Cyclostomi. In these groups it exists as a groove in the ventral wall of the pharynx, called the "hypobranchial groove." A similar groove is found in all tunicates (the "endostyle"), and is glandular in function. In the lower true vertebrates, where the paired components first arise, they remain distinct; in mammals all components fuse.
trIsornAuus AND STOMACH. 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, only 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 without 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 pyloric valve. The opening of the bile duct 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 enterou, and, coincidently, the entire alimentary tract, which is primitively straight, undergoes a great increase in length and becomes strongly folded. A differentiation of the prosenteron into (esoph agus and stomach is first indicated in selachians, and becomes pronounced in Amphibia. The two organs differ not only in their diameter, but also in the character of the mucous membrane, which is smooth and forms a ciliated epithelium on the (esophagus and folded and nominated in the stomach. In birds the (esophagus 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 proucntrieuhis, 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 complex 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 psalterium, 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 abomvum, 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 nivo•es is very great. The shorter the time they are exposed in the open held the less will be the chance of their destruction. It has therefure 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. 'Flue 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 and 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 (esophagus and is led by 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.