22 6 TEXT-BOOK OF PHYSIOLOGY. 



readily assimilable by the blood. That such a reconversion is neces- 

 sary would appear from the fact that the introduction of peptones 

 even in small amounts into the blood is followed by their elimination 

 unchanged in the urine. When injected into the blood in large 

 amounts, they act as toxic agents, giving rise to a fall of blood-pres- 

 sure, a diminished coagulability of the blood, coma, and death. 



After passing through the epithelium into the spaces of the villi 

 they are removed by the blood-vessels and carried direct to the liver. 

 Even though there is no appreciable increase in the amount of pro- 

 teid in the portal blood during digestion, there is every reason to 

 think that this is the route by which it reaches the general circulation. 

 Ligation of the thoracic duct does not interfere with proteid absorp- 

 tion nor with the normal elimination of urea nor with the weight of 

 the animal. 



Absorption of Fat. As previously stated, there are two views as 

 to the changes which fats undergo during digestion. According as 

 the one or the other is accepted will depend the view as to the nature 

 of the absorptive process. If it be assumed that the final stage in 

 the digestion of fat is a purely physical one, the production of an 

 emulsion in which the fats present themselves as fine granules, it is 

 difficult to give any satisfactory explanation of the mechanism by 

 which the epithelial cells take them up. Various theories have been 

 advanced to explain the process, but none are free from serious ob- 

 jections. This view of fat absorption has largely been based on the 

 observation that during digestion fatty granules can be seen in all 

 portions of the cell apparently passing toward the interior of the 

 villus. If, on the contrary, it be admitted that the final stage in the 

 digestion of fats is. the formation of soaps and glycerin, both of which 

 are soluble, their absorption can more readily be accounted for. 

 According to this view, the soaps and glycerin are again synthetized 

 by a process the reverse of that which is produced by the pancreatic 

 enzyme, with the appearance of minute granules of fat. That this 

 is the more probable view as to the mechanism of fat absorption is 

 evident from the fact that when animals are fed with alkaline soaps 

 and glycerin, or with fatty acids alone, globules of fat are found in 

 the epithelial cells and in the interior of the villus. 



With the passage of the fat-granules into the interior of the villus 

 they at once enter the lymph-radicle and become constituents of 

 the lymph-stream, to which they impart a white, milky appearance. 

 If the abdomen of an animal in full digestion be opened, the lymph- 

 vessels of the mesentery present themselves as distinct white threads. 

 An examination of the fluid they contain, known as chyle, shows the 

 presence of fat-granules of microscopic size. With the passage of 

 the chyle into the thoracic duct it also presents the same milky ap- 

 pearance. For this reason the lymphatics of the mesentery were 



