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. Tn the earth-inhabiting Gymnophiona and Amphiskena 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 Petromyzon and some of the sharks which lead an active carnivorous life, but 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 the 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 rtanoids. Teleosts lack such a spiral valve, but the absorbing sur face is increased by another means—namely, by out - pocketings, so - called pyloric appendages. That these are primarily not glands, but have an absorbing function, is indicated by two facts: (1) They are sometimes found stuffed with food, and (2) their presence seems to be correlated with the absence of the spiral valve and rice 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 dostens. 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 swim cases the pyloric appendages are indeed gland ular—as, for instance, when several open into the mesenterun by a common duet. Histological studies are needed to settle this question.
The Jlesenteron.—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 bus, in the frog, the anterior part of the meseuteron is covered by a fine network of folds. Further posteriorly these 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—rilli—produced into the lumen of the gut. Into these folds of the mucous epithelium are continued the connective tissue of the sub mucosa, together with blood vessels, lymph ves sels, and nerves. Food in solution is taken up by the epithelial cells just as an amoeba takes it up by throwing out. pseudopodia. A large share of the absorptive process is probably to be assigned to the lymph cells, which wander about in the submucosa and even make their way through the mucosa into the lumen of the gut.
Ifetenteron.—Like the other parts of the ali mentary tract, this becomes differentiated from the common enteron only in the higher verte brates. In the higher fishes it is indicated by an enlargement of the intestine. This enlarge ment 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 metenterow is enlarged to form a (functional) urinary bladder. In Amniota the metenteron is separated from the mesenteron by an ileo-ereeal valve (q.v.). In nearly all verte brates the metenteron—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 aseendens, transversus, and deseendens. A blind pocket cavum is often formed in connection with the metenteron. This is a mere swelling in the wall in reptiles. but attains an enormous devel opment 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 lie 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 developed. It would seem to be somewhat compensatory with relation to the rest of the meteuteron, 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 circuit] is greatly reduced, forming the vermiform process. In man the menu) is at first of nearly uniform char aeter—the vermiform process arises by a degen eration of its distal end—a process which occurs relatively late. This indicates that in man the cmcnin was quite recently of relatively greater importance, and indicates further that man's ancestors were herbivorous—a fact which the presence of the now degenerating third (hinder most ) molar likewise confirms.
There are certain other appendages of the me tenteron to which we can only refer. Such are the unpaired finger-shaped gland of the dorsal roof of the rectum in sharks, the paired dorsal pockets of Chelonia, and the unpaired bursa Fahricii of birds. The function of the last two is doubtful. 1Ve will digress to describe the bursa Fahrieii. This is a spherical or club shaped organ lying ventrad to the vertebral eol nmn and dorso-eaudad to the rectum, to which it is attached posterio-ventrad to the urogenital opening. It arises as a solid mass, in which sec ondarily cavities appear, lined by epithelium from the mueosa of the metenteron. They are, therefore, not to be regarded as lymph spaces, nor the organ as a lymph organ. Its develop ment is, therefore, much like that of the thymus gland. The organ degenerates toward the end of the first year, but persists throughout life in some species as an organ covered with a connec tive tissue coat, and possessing many elongated follicles lined by epithelium within. The func tion and phylogenctic significance of this organ are both unclear. Possibly it is homologous with the paired pockets of Chelonia ; the ontogeny of these latter organs is, however, yet quite un known.