THE SMALL INTESTINE. 89 



laris mucosae B, from which the villi arise, separates the mucosa from 

 the submucosa C. The horizontal line at the bottom of the diagram 

 indicates the outer limit of C and the beginning of the circular mus- 

 cular coat of the intestine. The villi, everywhere covered with 

 columnar epithelium, are represented in the drawing as widely sepa- 

 rated, but in the gut they are so closely studded as to afford but nar- 

 row chinks (crypts) between the prominences. In the interior of 

 each villus is a fine network of Uood capillaries (G G). The cells on 

 the borders of the villi secrete certain fluid material from the blood 

 circulating in the capillary plexuses, and pour it out into the crypts. 

 The crypts becoming filled with the fluid, the latter overflows and 

 passes into the lumen of the gut, to act in promoting digestion* 

 This is one source of the succus entericus, and there'is yet another. 



D-- 



FIG. 66. DIAGRAM SHOWING PORTIONS OF INTESTINAL Mucous MEMBRANE, CONCERNED IN THE 

 SECRETION OF THE Succus ENTERICUS. 



A. The mucosa. 



B. Muscularis mucosse. 



C. Submucosa. 



D. D, D. Villi. 



E. F. Crypts of Lieberkuhn. 



G, G. G. Blood plexuses of villi. 



H, K. Large vessels of submucosa, supplying the epithelium covering the villi. 



I. Neck of a gland of Brunner. 



J, J, J. Gland of Brunner in the submucosa. The secretion is emptied into the crypts as at F. 



From the bottom of some of the crypts, tubes will be found which, 

 piercing the muscularis mucosse, reach the submucosa where they 

 branch, become convoluted, are lined with secreting cells, and are 

 known as the glands of Brunner. These glands, which are practi- 

 cally elongated crypts, are surrounded by blood capillaries, and the 

 gland-cells secrete a fluid which is poured into the gut at the base of 



