Fig. 302 shows the intestine mucous membrane of the boa constrictor with well-developed and prominent villous projections.

Fig. 300.—Small intestine, of polar bear, Ursus maritimus. Mucous surface. (Columbia University Museum, No. 782.)   Fig. 301.—Duodenum, with entrance of pancreatic and biliary ducts and well-developed diverticulum Vateri in the cassowary, Casuarius casuarius (Columbia University Museum, No. 1821.)   Fig. 302.—Mucous membrane of midgut of Boa constrictor. (Columbia University Museum, No. 1837.)

Fig. 303.—Mucous surface of smallintestine of a species of African antelope,Cervicapra arundinacea. (Columbia UniversityMuseum, No. 1843.)

Fig. 304.—Mucous surface of smallintestine of Phocæna communis, porpoise.(Columbia University Museum, No. 1057.)

Fig. 305.—Mucous membrane of mid-gut of Lophius piscatorius, the angler, 18 cm.caudad of pylorus. (Columbia University Museum, No. 1838.)

Figs. 306, 307.—Intestinal mucous membrane of logger-head turtle, Thalassochelys caretta.(Columbia University Museum, No. 1839.)

Fig. 306.—Mid-gut.

 

Fig. 307.—End-gut.

Fig. 308.—Adult human subject. Mucous membrane of pyloro-duodenal junction and of duodenum. (Columbia University Museum, No. 1840.)   Fig. 309.—Adult human subject. Mucous membrane of small intestine, showing arrangement of valvulæ conniventes in successive portions of jejunum and ileum. (Columbia University Museum, No. 1841.)

Some birds, such as the snipes, herons and crows, have in place of the intestinal villi projecting folds of the mucosa, often arranged in a reticular manner. This type is prevalent in amphibia and fish ([Fig. 112], distal segment of midgut). Collections of lymphoid tissue in the mucous membrane of the small intestine, either aggregated to form Peyer’s patches ([Fig. 309]) or as solitary follicles, are only found in the two highest vertebrate classes, birds and mammals. In the former they appear scattered over the surface of the mucous membrane, in the latter they may be arranged in aggregations or regular rows. They are not secreting structures, but their exact function in absorption is not known. This lymphoid or adenoid tissue in certain forms is especially well developed at the ileo-colic junction, forming the lymphatic sac of some rodents, as lepus (cf. [Fig. 386]). It is not confined to the small intestine, but is found in the large intestine as well. At times it appears especially well developed in the terminal portion of the cæcal pouch (appendix), as in Lepus ([Fig. 388]).

The valvulæ conniventes or valves of Kerkring of the human small intestine serve to very greatly increase the secreting and absorbing mucous surface. They are not found in this complete development in any other mammals, although a very few forms present a transverse reduplication of the intestinal mucosa and the circular layer of muscular fibers. An example of this is found in the intestinal mucous membrane of a species of antelope, shown in Fig. 303.

The complete development of the valvulæ conniventes in man is possibly also associated with a mechanical function in connection with the upright posture. In some mammalia, as in certain rodents and the porpoise (Fig. 304), the mucous membrane of the terminal part of the small intestine is thrown into longitudinal folds.

The mucosa of the midgut in the lower vertebrates may be smooth, or thrown into longitudinal folds, or the longitudinal folds may become connected by oblique and transverse secondary folds, resulting finally in a more or less complicated reticulated pattern of crypts. A very good example of the type-form from which the more complicated conditions are derived is seen in Fig. 305, showing the mucous membrane of the midgut in Lophius piscatorius, the angler. The specimen is taken 18 cm. from the pylorus and shows a ground plan of longitudinal plicæ connected by short oblique cross folds.