ABSORPTION. 



257 



mucous; but, on contact with this fluid, even four hours 

 after death, the mucous becomes white and thicker and more 

 resisting. On examining it closer, we find that these phe- 

 nomena are at first only caused by changes in the epithelium. 

 The epithelial cells, which, when the animal is fasting, are 



Fig. 69. Intestinal villosities observed during absorption (especially during 

 the absorption of fat). (Virchow.)* 



small, somewhat diffluent, and hardly forming a distinct 

 membrane, swell when excited by the gastric juice ; and, as it 

 were, standing erect, become three times their original size, 

 forming a resisting membrane, which may almost be dis- 

 sected ; the villi are then pressed against each other, the epi- 

 thelium forming four-fifths of their bulk. The epithelial cells 

 also change in color, becoming whitish ; this seems to be due 

 to the large number of drops of fat found inside them, and 

 the same phenomenon takes place even when the fluid of 

 the stomach, which is brought in contact with the mucous, 

 contains no fat. We know, however, that every cell con- 

 tains fat ; this fat, it is true, is disguised, but it becomes visi- 

 ble under certain circumstances, especially when an interior 



* A, Intestinal villosity of man, taken from the jejunum. la a we see the 

 columnar epithelium, with its nuclei, continuing as far as the surface of the 

 villosity. c, Central chyliferous duct, v, , Blood-vessels. The embryonic 

 nuclei of the connective tissue are seen in the remaining part of the body of the 

 villositv. 



B, Villosity of a dog, contracted. 



O, Villosity of man during intestinal absorption, the fat becoming a part of 

 the body of the villus itself. In D a large collection of fat is seen. (280 

 diam.) 



17 



