290 THE DIGESTIVE SYSTEM 



The long axes of the ovoid nodules exceed the average thick- 

 ness of the mucous membrane so that the patch forms a superficial 

 elevation of the mucosa and its deeper surface penetrates the 

 muscularis mucosae and enters the submucous coat. Hence occa- 

 sional fragments of the muscularis mucosae often occur between 

 the bases of the constituent nodules ; villi are found upon the free 

 surface of the follicles only in the intervals between the constitu- 

 ent nodules. The largest of the nodules lie near the center of the 

 patch, the smallest are found at its periphery. 



Above the level of the ileum the largest collections of lymphoid 

 tissue in the intestinal mucosa occur in the upper part of the duo- 

 denum, where there- are extensive infiltrations of dense lymphoid 

 tissue many of which contain typical nodules with germinal cen- 

 ters. These masses of lymphoid tissue are penetrated by the ducts 

 of Brunner's glands, whose secreting portions form a bed upon 

 which the lymphoid tissue rests. The duodenal patches differ 

 slightly from those in the ileum in that they form a more confluent 

 mass with relatively fewer nodules, possess a more diffuse charac- 

 ter, are more deeply situated, and are therefore covered by the 

 corium of the mucosa which contains both crypts and villi. 



THE INTESTINAL VILLI are long finger-like projections 

 which vary much in form in different mammals and in different 

 portions of the tract in the same individual. They are, perhaps, 

 most highly developed in the dog, where they form long projec- 

 tions with expanded or clubbed extremities and a constricted base 

 or neck. 



In man the villi are of a more conical shape, the base being, as 

 a rule, slightly broader than the free extremity. In the duode- 

 num of man they possess a foliate shape, in the ileum they are 

 conical or somewhat clavate. 



The villus is formed by a projection of the corium which is 

 covered by the lining epithelium of the intestine. The axis of the 

 villus contains a large lymphatic capillary or lacteal, which begins 

 in the inner third and proceeds outward through the corium to 

 enter a lymphatic plexus lying just within the muscularis mucosae. 

 In the base or outer portion of the villus the lacteal is surrounded 

 by small groups of smooth muscle fibres which are disposed in an 

 axial direction, and which are ontogenetically derived from the 

 muscularis mucosae. Many of these fibres turn outward and are 

 attached to the basement membrane beneath the epithelium at 

 the sides and tip of the villus. By their rhythmic contraction 



