438 CHEMISTR 1 r OF THE DIGEST/ 1 E PR O CESSES. 
during proteid digestion. Such evidence can only give a reliable 
estimate of the amount formed relatively to other proteid products, 
when the rate of absorption from the intestine of these various products 
is known. 
Kolliker and Midler 1 found only microscopic traces of leucine and 
tyrosine in the upper part of the small intestine of carnivorous animals, 
and none in the lower part. Kiilme 2 subjected fibrin to tryptic digestion 
in a tied-off loop of intestine, into which the pancreatic duct entered, 
and subjected the residue to analysis after four hours. Leucine and 
tyrosine were found among the products, but the yield was small, and, 
moreover, the conditions in such an experiment are not quite comparal >le 
to those of natural digestion. Indeed, Kiihne himself thinks it probable 
that the greater part of the "peptone" being rapidly absorbed escapes 
such a decomposition. 
Schmidt-Mulheim 3 states that, in proteid digestion in the carnivora, leucine 
and tyrosine are either not formed at all, or else in such small quantities that 
their absorption is of no physiological importance as a means of removing from 
the alimentary canal any appreciable amount of nitrogen derived from the 
proteid foodstuffs. 
On the other hand, Sheridan Lea 4 obtained from the intestinal contents of 
a dog, six hours after a plentiful meal of lean flesh, what must be regarded as a 
considerable amount of amido-acids to be found there at any given time (par- 
ticularly when it is remembered that the amount of intestinal contents in the 
dog at any given moment, even during active proteid digestion, is very scanty), 
namely, 1 grm. of pure leucine, and "3 grm. of tyrosine. These figures equal the 
total amounts obtainable of these products from 10 grms. of dried proteid, and 
if it be assumed that leucine and tyrosine are absorbed with a rapidity equal to 
that with which albumoses are taken up, indicate that a considerable percentage 
of proteid was being converted into amido-acids, and absorbed as such. 
The relative amount of proteid decomposed in the intestine into amido- 
acids, as well as that absorbed in the various other forms, probably varies within 
wide limits with the state of nutrition of the animal and the amount of proteid 
food. It is possible, as Foster 5 states, that such a degradation of proteid in 
the intestine may serve as a safety-valve to the economy, diverting from the 
tissues the burden of an often unnecessarily large proteid metabolism. The 
waste of energy to the animal economy caused by the disintegration of proteid 
into amido-acids in the intestine is often advanced as an argument against the 
occurrence of this process to any marked extent. Certainly the potential 
chemical energy of the proteid is lost to the economy, as far as the performance 
of some forms of physiological work is concerned, but it should not be forgotten 
that the total amount of energy abstracted by the animal from its food is 
measured by the chemical form in which it enters and that in which it leaves 
the body, and a given portion of proteid entering the body and then leaving it 
as urea, water, and carbon dioxide, will give up to the body exactly the same 
store of energy, no matter what may be the intermediate steps by which it is 
reduced from one form to the other. In one case the energy is set free in 
the tissues, in the other in the intestine. In the second case, the heat 
set free by chemical decomposition is communicated to the intestine and 
carried off by the circulating blood, to keep up the temperature of the body, 
thus sparing reserve chemical energy which would otherwise have to be used 
for this purpose. 
1 Verliandl. d. phys.-med. Gesellsch. zu Wihrzburg, 1856, Bel. vi. S. 499. 
- Virehow's Arckiv, 1867. Bd. xxxix. S. 130. 3 Loe. ciL 
4 Joum. Physiol., Cambridge and London, 1890, vol. xi. ]>. 255. 
5 "Text-Book of Physiology," 18S9, 5th edition, part ii. p. 476. 
