THE SMALL INTESTINE. 



l6 35 



epithelium covering the free surface consists of a single layer of cylindrical cells which 

 exhibit a striated cuticular border next the intestinal lumen. This border lacks 

 stability, and is resolvable into minute prismatic rods, placed vertically and probably 

 continuous with the spongioplastic threads within the body of the cell. In many 

 places, especially over the villi, mucus-producing goblet-cells share the free surface 

 with the ordinary epithelial elements. Between the latter migratory leucocytes are 

 always to be seen. The stroma or tunica propria of the mucous coat resembles 

 lymphoid tissue, being composed of a connective-tissue reticulum containing numerous 

 small round cells similar to lymphocytes. This stroma fills the spaces between the 

 glands and forms the core of the villi over which the epithelium stretches. The deep- 



FIG. 1384. 



Villus 



Duct of Brunner's glands 



Muscularis mucosae 



& 



m 



Brunner's glands W 



rifice of gland of Lieberkiihn 



Brunner's glands 



Serous coat 



Circular muscle 



Longitudinal muscle 



Transverse section of small intestine (lower part of duodenum), showing general arrangement of coats. X 90. 



est part of the mucous coat is occupied by a well-marked muscularis mucosce, in 

 which an inner circular and an outer longitudinal layer are distinguishable. 



The villi are minute projections of the mucous surface, barely visible to the un- 

 aided eye, the presence of which imparts the characteristic velvety appearance to the 

 inner surface of the small intestine. Although found throughout the latter, from the 

 pylorus to the ileo-colic valve, they are most numerous (from 20-40 to the sq. mm.) 

 in the duodenum and jejunum and less frequent (from 1530 to the sq. mm.) in the 

 ileum. In the duodenum they appear immediately beyond the pylorus, but reach 

 their best development in the second part, where they measure from . 2. 5 mm. in 

 height and from .3-1 mm. in breadth ; they are, therefore, here low and broad. In 

 the jejunum the villi are conical and somewhat laterally compressed, while in the 

 ileum their shape is cylindrical, filiform, or wedge-like, their length and breadth 

 being from .5-1 mm. and from .2 .4 mm. respectively. The villi are projections of 



