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STUDIES IN ADVANCED PHYSIOLOGY. 



brane are small glands called the crypts of Lieberkuhn. 

 The epithelial lining that covers the mucous membrane is 

 one-layered. This epithelium extends down into the crypts 

 of L,ieberkiihn and forms in them the secreting cells. It is 

 also continued over the villi. A single villus, therefore, 

 consists of a layer of epithelium covering the fibrous cen- 

 tral portion, through which run numerous blood-vessels, 

 while in the center of the villus there is a single large lym- 

 phatic called here a lacteal. The rich supply of blood- 

 vessels to such a villus accounts for the efficiency of these 

 structures in the process of absorption, while the central 

 lacteal is the avenue through which the fats reach the body 

 in a manner to be described in the succeeding chapter. 



Fig. 114. INJECTED VILLI OF THE HUMAN INTESTINE. (After Teichmann.) 

 a. b, lacteals (white); c, horizontal lacteals; d, networks of blood-vessels (dark). 



Nerves and bits of plain muscular tissue distributed through 

 the body of the villus also occur. These bits of plain 

 muscular tissue make possible the slight powers of con- 

 traction which are said to materially aid in their absorptive 

 capacity. 



The villi extend from the beginning of the duodenum 

 through the length of the small intestine. The crypts of 

 Ivieberktilm between them have a similar distribution. In 



