716 PHYSIOLOGY OF DIGESTION AND SECRETION. 



cent. The amount of fat that may be lost in the feces varies also 

 with other conditions. If, for instance, an excess is taken with the 

 food or if the bile flow is diminished or suppressed the percentage 

 in the feces is increased. The usual amount of fat allowed as a 

 maximum in dietaries is from 100 to 120 gms. daily. 



Absorption of Proteids. Most of the experimental work on 

 record shows that the digested proteids are absorbed by the blood- 

 vessels of the villi, although after excessive feeding of proteid a 

 portion may be taken up also in the lymphatics.* This accepted 

 belief rests upon two facts: First (Schmidt-Mulheim), if the thoracic 

 duct (and right lymphatic duct) is ligated, so as to shut off the lym- 

 phatic circulation, an animal will absorb and metabolize the usual 

 amount of proteid, as is indicated by the urea excreted during the 

 period. Second (Munk), if a fistula of the thoracic duct is established 

 and the total lymph flow from the intestines is collected during the 

 period of absorption after a diet of proteid, it is found that there is no 

 increase in the quantity of the lymph or in its proteid contents. 

 The form in which proteid is absorbed remains, however, a mystery. 

 Under normal conditions the proteid food is digested by the success- 

 ive actions of pepsin, trypsin, and probably erepsin. During this 

 digestion peptones and proteoses are formed and may be absorbed as 

 such, or they may be further broken down by trypsin and erepsin to 

 the amido-bodies, leucin, tyrosin, arginin, etc., and the intermediate 

 compounds, the polypeptids or peptoids (see p. 708), and be absorbed 

 in the form of these split products. Examination of the blood does 

 not show, however, the presence of any of these bodies, and in spite 

 of the fact that the process of absorption is long continued and the 

 total amount of absorbed products in any given specimen of blood 

 may therefore be very small, it is perplexing not to be able to ob- 

 tain indubitable proof of the existence in the blood of some of the 

 products of digestion. Several possibilities have been suggested. 

 It is conceivable that the peptones or the more simple split products 

 may be synthesized in the wall of the intestine or in the liver to 

 the proteids of the blood, the serum-albumin or globulin; it is possi- 

 ble that many of the end-products of the digestive splitting may be 

 further oxidized and converted to urea in the liver and only a 

 fractional part be really synthesized into the proteids of the body, 

 or it is possible that the absorbed proteid exists in the blood in 

 some special form not as yet recognized. The whole question is 

 evidently one that can not be discussed very profitably at present ; 

 it awaits the results of further investigation. In this connection 

 attention should be directed to the fact that many forms of proteid 

 may be absorbed apparently without previous digestion. This fact 

 has been demonstrated for isolated loops of the small intestine and 

 *See Mendel, "American Journal of Physiology," 2, 137, 1899. 



