1. It provides space for the retention of partly digested substances, and of such as are difficult of digestion, mixed with the secretions of the preceding intestinal segment, until the digestive elaboration is completed.

2. It increases the intestinal mucous surface for absorption, and may develop, in certain cases, special localized areas of lymphoid tissue.

These two functional characters may be shared by other segments of the intestinal tract, which undergo corresponding structural modifications. It is only necessary to refer in this connection to the extreme morphological variations encountered in the stomach. The intestinal canal proper, however, in many instances exhibits structural peculiarities which possess the functional significance of the cæcal apparatus. Thus the projection into the lumen of the canal of a series of mucous folds, or the development of a continuous spiral mucous valve, evidently serves the double purpose of prolonging the period during which the intestinal contents are retained, and of increasing the intestinal mucous surface for absorption.

Fig. 467.—Galeus canis, dog-shark. Alimentary tract opened, showing spiral intestinal valve. (Drawn from Columbia University Museum preparation No. 1429.)   Fig. 468.—Ceratodus forsteri, Australian lung-fish. Intestinal canal with spiral valve. (Columbia University Museum, No. 1645.)

This spiral mucous fold is encountered in the straight intestinal canal of the Cyclostomata ([Fig. 465], IV, 1, and [Fig. 310]), Selachians ([Figs. 466] and 467) and Dipnœans (Fig. 468). Phylogenetically it is a very old structure, for evidences of its existence are found in the fossil remains of some Elasmobranchs. In the Ostrich ([Fig. 341]) the enormously developed cæca possess the same spiral mucous fold in the interior. The direct combination of the cæcum and spiral fold is again seen in certain mammalia, as in Lepus ([Fig. 387]). In some Ophidians the same physiological purpose is served by the manner in which the convolutions of the long intestine are bound together by a subperitoneal arachnoid membrane. The lumen of the canal is thus made to assume a spiral course ([Figs. 331] and 469). The mucous folds of the human intestine, both the valvulæ conniventes and the crescentic folds of the large intestine, represent the same spiral valve, perhaps modified and influenced by the erect posture of man ([Figs. 470]-[475]).

Fig. 470.—Human small intestine, opened toshow valvulæ conniventes. (Columbia UniversityMuseum, No. 1841.)

Fig. 469.—Python molurus, Indian python. Mid-gut, distendedand fenestrated to show spiral course of lumen.(Columbia University Museum, No. 725.)

Fig. 472.—Human large intestine,opened and in section, showing colicplicæ. (Columbia University Museum,No. 1847.)

Fig. 471.—Human large intestine,showing colic tænia and plica.(Columbia University Museum, No.1848.)

Fig. 473.—Cynocephalus anubis, olive baboon. Large intestine, with cross-section showing colic tænia and plicæ. (Columbia University Museum, No. 26/1168.)   Fig. 474.—Comparison of portion of human transverse colon with distal segment of rabbit’s large intestine, showing same arrangement of longitudinal muscular bands (colic tænia) and colic sacculations. (Columbia University Museum, No. 1589.)
Fig. 475.—Felis leo, lion. Large intestine, with transverse sec­tion, showing smooth carnivore lumen, without sac­cu­la­tions or plicæ. (Columbia University Museum, No. 1600.)   Fig. 476.—Pyloric cæca of Gadus callarias, codfish. (Columbia University Museum, No. 1825.) A. Bound together by connective tissue and blood-vessels. B. Dissected to show confluence of cæca to form a smaller number of terminal tubes of larger calibre entering the intestine.

Fig. 477.—Alimentary canal of Accipenser sturio, sturgeon. Numerous pyloric cæca are bound together to form a gland-like organ.