120 II. DIGESTION AND ABSORPTION OF FATS 



at the different levels of the small intestine. This contrasts with the situa- 

 tion in mammals, where all of the small intestine presents a more or less 

 uniform structure. A comprehensive study of variations in structure of 

 the pigeon's intestine has been made by Verzar and McDougall. 554 Marked 

 differences in the structure of the duodenum, jejunum, and ileum can be 

 noted, not only macroscopically but also microscopically. The variations 

 in anatomy are evident from the calculations summarized in Table 16 

 (p. 118). 



About one-third of the total intestinal surface occurs in the duodenum 

 while, in the jejunum, the surface continually decreases, so that the last 

 third has onlv about one-half of that of the first third; the ileum, which is 





/i. 



1 w; ; .,'M r", I / 



Fig. 6. Intestinal epithelium from duodenum of a man 44 years of age. This 

 illustrates the trophosphongium (Golgi apparatus) and also a mucus-secreting 

 cell (at right). 595 



equal in length to one-third of the total jejunum, has less than one-half 

 the surface of the last third of this portion of the intestine. 



The villi vary in structure with the portion of intestine in which they are 

 found. Thus, in the duodenum, they are extremely long, narrow, finger- 

 like processes, which show strong pumping movements. In the jejunum, 

 the villi are shorter, but they also appear to undergo pumping movements. 

 The villi present in the ileum are short, tongue-shaped structures, closely 

 packed together; in contrast to the villi higher up in the small intestine, 

 those located in the ileum do not present the contractions described as 

 pumping. Figure 5 (p. 119) illustrates the variations in structure noted in 

 various sections of the small intestine of the pigeon. 



b. The Histological Structure of the Intestinal Mucosa. The mucosa 

 lining the intestinal cavity is of great importance in the absorption of food- 



