210 



INTESTINAL DIGESTION 



times bifurcated, and are composed externally of a structureless base- 

 ment-membrane, lined with a layer of cylindrical epithelium resembling 

 the cells that cover the villi, but shorter. Mixed with these cells are 

 the so-called goblet-cells, which will be described in connection with the 

 intestinal villi. The goblet-cells are thought to secrete mucus. They 

 exist in variable number, often being as abundant as the ordinary 

 epithelial cells. There is much difference of opinion in regard to the 



Fig. 43. Intestinal tubules, x 100 (Sappey). 



A. From the dog: i, excretory canal; 2, 2, primary branches; 3, 3, secondary branches; 4, 4, 

 terminal culs-de-sac. B. From the ox: i, excretory canal; 2, principal branch dividing into two; 3, 

 branch undivided ; 4, 4, terminal culs-de-sac. C. From the sheep: i, trunk; 2, 2, branches. D. Single 

 tube, from the pig. E. From the rabbit and hare : i, simple gland ; 2, 3, 4, bifid glands ; 5, com- 

 pound gland from the duodenum. 



office of the epithelium, which many histologists regard as non-secretory. 

 The lumen of the tubules often contains, however, a mucus-like secretion. 

 The length of the tubules is about equal to the thickness of the mucous 

 membrane and is about y 1 ^ of an inch (0.33 millimeter). Their diameter 

 is about -\- of an inch (0.07 millimeter). In man they are cylindrical, 

 terminating in a single rounded blind extremity, which frequently is a 

 little larger than the rest of the follicle (see Plate IV, Fig. 4). These 

 follicles present considerable differences in the lower animals. In some 

 they are bifid or branched, but in man nearly all are simple. They are 



