118 THE HUMAN MECHANISM 



In the structure of the inner or mucous membrane two 

 points are of importance to us. In the first place, numerous 

 simple tubular glands discharge into the intestinal tube an 

 important digestive juice, the intestinal juice ; in the second 

 place, fingerlike processes, or villi (0.50.7 mm. long by 

 0.1 mm. thick), arise from its surface and project into the 

 intestinal cavity. These are important organs of absorp- 

 tion. The entire surface of the villi, the glands, and the 

 plane surface of the intestine between these structures is 

 lined with a continuous membrane composed of colum- 

 nar cells, which separates blood vessels and lymphatics in 

 the intestinal wall from the cavity of the intestine (see 

 Fig. 59). The products of digestion must therefore pass either 

 through these cells or between them to enter the blood 

 or lymph. 



The intestine is some twenty or twenty-five feet in length, 

 and the intestinal glands (Fig. 58) constantly secrete intes- 

 tinal juice upon the contents as they are slowly moved along 

 the tube. Two other juices are added to the intestinal con- 

 tents almost immediately after then* entrance to the upper 

 part of the small intestine. These are the pancreatic juice 

 and the bile, which are secreted, respectively, by the pancreas 

 and the liver. The entrance of the ducts of these glands is 

 shown in Fig. 54. It is not necessary for our present purpose 

 to describe the minute structure of these organs; it is enough 

 for the student to understand that they are glands (p. 29) 

 which pour their secretions through ducts into the intestine 

 very much as the salivary glands pour their secretions into 

 the mouth. 



26. The mechanism of secretion of pancreatic juice, bile, 

 and intestinal juice. The mechanism which evokes the secre- 

 tion of the pancreatic juice has already been described 

 (p. 89). It will be recalled that the lining cells of the 

 intestine immediately beyond the pylorus (duodenum) con- 

 tain a material which when acted upon by the hydrochloric 



