George S. Huntington

“ In the former this segment of the canal functions as an accessory digestive apparatus and hence, as we will see, often develops accessory structural modifications, such as a large cæcum and spiral colon, while in the latter it acts almost solely as a canal for the evacuation of the indigestible remnants.
Again, the large intestine is better developed in the higher animals, in mammalia and to a lesser degree in birds, in whom the functional demands for nutrition are active and require that a relatively large amount of food should pass through the digestive tract in a given time. ”

“ To illustrate the arrangement which is found in the adult human subject the descending duodenum and pancreas should be allowed to fall over to the right so as to cover the vena cava and the mesal part of the ventral surface of right kidney. The adult human condition will now be produced if we assume that the structures are fixed in this position by the obliteration of the apposed serous surfaces, viz., the parietal peritoneum over kidney and vena cava on the one hand and the right layer of the mesoduodenum and the dorsal visceral peritoneum of the duodenum on the other. ”

“ This is especially the case in herbivora and omnivora, whose food is bulky, containing a large amount of refuse material, and is hence only slowly digested. On the other hand the food of the carnivora is easily and rapidly digested and absorbed. After passing through the small intestine hardly any substances remain which are capable of digestion and absorption. Hence the large intestine of herbivora and omnivora is uniformly longer in proportion to the small intestine than it is in carnivorous animals. ”

“ In structure the gland is usually acinous, resembling the salivary glands. It is well developed in the selachians, forming a triangular body connected with the beginning of the midgut (Fig. 202) . In some instances the gland elements do not extend beyond the intestine itself, but remain imbedded in the wall of the midgut, as in Protopterus. In certain adult teleosts the pancreas is surrounded by the liver (Fig. 196) , in others it does not appear as a compact gland but is distributed in the form of finely scattered lobules throughout the mesentery between the two layers of this membrane. ”

“ After opening the abdominal cavity of the human adult, elevating the liver and depressing the stomach, the hepatic attachment of the lesser omentum can be traced as already described. It will then be observed that the gastric attachment of the membrane lies in one plane following the lesser curvature while the hepatic attachment forms a broken line, with the angle situated at the left extremity of the transverse fissure. ”

“ The large intestine is drawn over so as to represent the human ascending and transverse colon in one segment, the descending colon in the other, and the mesocolon appears correspondingly as transverse and descending. In other words the cat’s intestines as arranged in the figure would represent the stage in the human development in which cæcum and beginning of large intestine are still subhepatic in position ventrad of the right kidney, before differentiation of ascending and transverse colon by descent of cæcum into right iliac fossa. ”

“ In order to obtain a correct conception of the adult human conditions it is finally necessary to consider the development of the spleen and pancreas in their connection with the dorsal mesogastrium and to note the changes which are produced by adhesion of portions of the great omentum to adjacent serous surfaces. ”

“ Animals, in which digestion is rapid and the usual food small in bulk and concentrated in its nutrient qualities, have a relatively short intestine, while the canal is longer in forms subsisting on food which is bulky and which demands considerable time for its elaboration. Hence we find the relatively shortest intestine in carnivora, the longest in herbivora, while the canal in omnivora occupies an intermediate position in regard to its relative length. ”

“ We have seen that the intestinal tube of all vertebrates is the product of two of the embryonal blastodermic layers, the entoderm and mesoderm. The former furnishes the characteristic and cardinal elements of the digestive tract, viz., the secretory and absorbing epithelium of the mucous membrane and of the accessory digestive glands, the liver and pancreas. ”

“ In order to complete the embryological history of the alimentary canal it is necessary to take brief account of another structure derived from it, namely the allantois. Its significance to the adult organism is seen in connection with the genito-urinary tract, the urinary bladder being formed by its persistent portion. In the embryo, however, it has important nutritive and respiratory functions. In the embryos of the higher vertebrates nutrition depends only in the earliest stages upon the yolk-sac of the ovum, over which a vascular network extends. ”