The _valvulae 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. [Illustration: FIG. 303.--Mucous surface of small intestine of a species of African antelope, _Cervicapra arundinacea_. (Columbia University Museum, No. 1843.)] The complete development of the valvulae 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_. [Illustration: FIG. 304.--Mucous surface of small intestine of _Phocaena communis_, porpoise. (Columbia University Museum, No. 1057.)] [Illustration: FIG. 305.--Mucous membrane of mid-gut of _Lophius piscatorius_, the angler, 18 cm. caudad of pylorus. (Columbia University Museum, No. 1838.)] 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 plicae connected by short oblique cross folds. [Illustration: FIGS. 306, 307.--Intestinal mucous membrane of logger-head turtle, _Thalassochelys caretta_. (Columbia University Museum, No. 1839.)] [Illustration: FIG. 306.--Mid-gut.] [Illustration: FIG. 307.--End-gut.] 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 valvulae 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. [Illustration: FIG. 308.--Adult human subject. Mucous membrane of pyloro-duodenal junction and of duodenum. (Columbia University Museum, No. 1840.)] [Illustration: FIG. 309.--Adult human subject. Mucous membrane of small intestine, showing arrangement of valvulae conniventes in successive portions of jejunum and ileum. (Columbia University Museum, No. 1841.)]