ABSORPTION. 219 



(that is unused for tissue repair and growth) fragments of protein 

 disintegration is doubtless varied; a large portion is undoubtedly ab- 

 sorbed by the epithelial cells of the villi and mucosa after which they 

 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 sugar or fat and subsequently utilized by the 

 organism for heat production. The dynamic portion of the amino-acid is 

 this deaminized remainder. Another portion is 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. 



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 

 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 micro- 

 scopic size. With the passage of the chyle into the thoracic duct it also 

 presents the same milky appearance. For this reason the lymphatics of 

 the mesentery were erroneously termed lacteals. The chyle as obtained 

 from these lymph-vessels possesses the same qualitative though not quanti- 

 tative composition as lymph, the difference being mainly in the large excess 

 of fat in the former. Indeed, chyle may be regarded as lymph with the ad- 

 dition of fat. 



