DIGESTION AND ABSORPTION IN THE INTESTINES. 737 



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 Proteins. Most of the experimental work on 

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

 vessels of the villi, although after excessive feeding of protein 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 protein as is indicated by the urea excreted during the pe- 

 riod. 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 protein, it is found that there 

 is no increase in the quantity of the lymph or in its protein contents. 

 The form in which protein is absorbed and circulates in the blood 

 is not satisfactorily determined. Under normal conditions the 

 protein food is digested by the successive 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 amino-bodies, leucin, 

 tyrosin, arginin, etc., and the intermediate compounds, the poly- 

 peptids or peptoids (see p. 729), and be absorbed in the form of 

 these split products. Some observers claim to have found peptones 

 or proteoses as a normal constituent of the blood, but this claim 

 has not been satisfactorily established. Others have shown the 

 presence of traces of the amino-acids,f but much uncertainty exists 

 as to the precise form in which the protein nourishment for the body 

 exists normally in the blood. Several possibilities have been sug- 

 gested. 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 proteins of the blood, the serum-albumin or globulin; 

 it is possible 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 proteins of the bod}' , 

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

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

 evidently one that cannot 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 protein 

 may be absorbed apparently without previous digestion. This fact 



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

 t For references see Howell, 'American Journal of Physiology," 1906, 

 xvii., 273. 



47 



