260 



HISTOLOGY 



bile and pancreatic ducts enter the descending portion of the duodenum. 

 Beyond this point they become fewer, and disappear before the end of 

 the duodenum is reached. Except for these glands the duodenum is 

 essentially like the remainder of the small intestine, described in the 

 following section. 



JEJUNUM AND ILEUM. 



The lining of the small intestine, including the duodenum, has a velvety 

 appearance, due to the presence of innumerable cylindrical, club-shaped 

 or foliate elevations, known as villi (hairs or nap). True villi are found 

 in the large intestine of the embryo but they disappear before birth; they 

 are said to occur also in the pyloric end of the stomach, but it is question- 

 able whether these are typical villi or merely irregular folds. Elsewhere 

 in the digestive tube, villi are absent. At the bases of the villi there are 

 simple tubular pits of glandular epithelium, which extend to the muscu- 

 laris mucosae but do not penetrate it; these are the intestinal glands (glandu- 

 le intestinales, formerly known as crypts of Lieberkiihn). An enlarged 



PIG. 253. 



A, Surface view of the hardened mucosa of the small intestine (after Koelliker). B, Side view of a wax 

 reconstruction of the epithelium in the human duodenum (Huber). i. g., Intestinal gland; v., villus. 



surface view of the hardened mucous membrane is shown in Fig. 253, A. 

 The orifices of the glands appear as round holes; the villi, which are from 

 0.2-1.0 mm. in height, have fallen over in various directions. Within 

 the duodenum the villi are low leaf-like folds, 0.2-0.5 mm. high, seen in 

 side view in the reconstruction, Fig. 253, B. Their shape cannot be deter- 

 mined from inspecting single sections (cf. Fig. 251). 



It will be seen that villi are essentially circumscribed folds, and they 

 have been said to arise through the subdivision of longitudinal ridges 

 (Berry, Anat. Anz., 1900, vol. 17, pp. 242-249). According to Johnson 

 (Amer. Journ. Anat., 1910, vol. 10, pp. 521-561) they develop as low 

 knob-like elevations which increase in height. They may become sub- 

 divided, as indicated by bifid villi (Fig. 253). 



The small intestine contains other elevations of its lining which are 

 much larger than the villi. These are the circular folds (plica circulates, 



