222 TEXT-BOOK OF PHYSIOLOGY 



which it passes to enter the blood of the general circulation. Ligation of 

 the thoracic duct does not interfere to any appreciable extent with protein 

 absorption nor with the normal production and elimination of urea, nor with 

 the weight of the animal. 



Many facts in the physiologic chemistry of the body raise the question 

 as to what percentage of the amino-acids produced in the intestine daily is 

 utilized for tissue repair and growth. If the protein requirements of Chit- 

 tenden, viz.: 58 to 60 grams only, are necessary for repair and growth, 

 then approximately one-half the amino-acids produced from the protein 

 usually consumed must be disposed of in some other manner. The manner 

 of disposal of these unused (that is, unused for tissue repair and growth) 

 fragments of protein disintegration is doubtless varied; some of the amino- 

 acids, after absorption by the epithelial cells of the villi and mucosa, are 

 deprived of NH 2 (the amino-acid nitrogen) or deaminized; the NH 2 is then 

 converted into ammonia, combined with carbon dioxid to form ammonium 

 carbonate, carried to the liver, and changed into urea. That this is very 

 probably the case is rendered likely from the presence of a large quantity of 

 ammonia in the mucous membrane of the intestine and in the blood of the 

 portal vein, in which after a meal rich in protein it may be four times as 

 great as in the arterial blood. The remainder of the amino-acid molecule is 

 changed into some carbonaceous radical and finally into sugar or fat and 

 subsequently utilized by the organism for heat production. The dynamic 

 portion of the amino-acid is this deaminized remainder. Other of the surplus 

 amino-acids are acted on by intestinal bacteria, and converted into 

 simpler compounds, after which they are eliminated in the feces or absorbed 

 and carried to the liver where they undergo other changes and eventually 

 appear in the urine. 



The ammonia set free during digestion is absorbed, carried to the liver 

 and transformed to urea. 



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 objections. This view of fat absorption has largely been based on 

 the observation that during digestion fatty granules can be seen in all por- 

 tions 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 synthesized by a process the reverse of that 

 which is ^ brought about by the pancreatic enzyme, with the appear- 

 ance 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 



