Fig. 306, showing the midgut mucosa of the loggerhead turtle (Thalassochelys caretta), exhibits the same arrangement further developed, resulting in a fine reticulated pattern, while in the endgut of the same animal the primitive longitudinal folding is resumed (Fig. 307).
The number and size of the human valvulæ conniventes vary in different parts of the small intestine ([Fig. 309]). They are not usually found in the beginning of the duodenum ([Fig. 308]), but commence in the second or descending portion.
They become very large and closely packed immediately beyond the common entrance of the biliary and pancreatic ducts and continue to be well developed and numerous throughout the rest of the duodenum and upper half of the jejunum ([Figs. 308] and [309]). From here on they become smaller, more irregular and less closely packed, and finally in the terminal two feet of the ileum disappear almost entirely ([Fig. 309]). This varying development of the valvulæ is the chief reason why a given segment of the ileum weighs less than a corresponding length of the jejunum. This reduction in the fold-formation of the intestinal mucosa toward the terminal portion of the midgut is seen even in the lower vertebrates. Thus in [Fig. 112], showing the entire intestinal tract of the conger eel, Echelus conger, in section, the plicæ of the mucous membrane in the proximal segment of the midgut, at and immediately beyond the entrance of the biliary duct, are prominent and numerous. This redundancy continues but slightly reduced in the descending limb of the intestinal loop, while in the ascending limb and up to the ileo-colic junction the folds are reduced to a fine reticulated meshwork. Beyond the ileo-colic valve plate, in the short endgut, the mucosa again presents numerous pointed reduplications.
II. ENDGUT OR LARGE INTESTINE.
In this segment of the intestinal canal the undigested remnants of the food are collected and evacuated from time to time.
In addition, the mucous membrane of the large intestine absorbs all digested material which is passed from the small intestine. While digestion of food-substances will not be inaugurated in the large intestine, material already in the process of digestion and mixed with the intestinal juices of the preceding segment, will be further elaborated in this portion of the canal and the nutritive products absorbed. 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 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. On the other hand in the lower cold-blooded vertebrates the metabolism is less active, less food is taken and it is not necessary to secure all the nutrient material contained in the same for the organism. The great differences observed in the vertebrate series in regard to length, width and structure of the large intestine depend upon these physiological conditions. The divisions of the human large intestine into cæcum, ascending, transverse and descending colon, sigmoid flexure and rectum are found only in the primates, and here not uniformly.
In the lower vertebrate classes the endgut is very short, corresponding only to the pelvic segment of the Mammalia (rectum), a colon proper being absent in these forms (cf. [Fig. 112], Echelus conger). The human large intestine exhibits a very characteristic structure. Throughout the greater part of the colon the longitudinal muscular layer is mainly disposed in the form of three bands or tænia (ligamenta coli). The canal itself is longer than these bands, thus producing a folding of the walls in the form of three rows of pouches (cellulæ coli), in the intervals between the bands. The pouches of each row are separated from each other externally by constrictions, internally by projecting crescentic folds (plicæ coli) ([Figs. 471], [472] and [474]).
This arrangement of the large intestine is also found in the monkeys ([Fig. 473]) and in certain Rodents ([Fig. 474]).