244 DIGESTION AND ASSIMILATION OF THE FOODSTUFFS 



fore stored up in the tissues. There is a limit, however, to the capacity 

 of the tissues to retain amino-acids and this upper limit of capacity 

 varies with different tissues. Thus the upper limit in the case of 

 muscular tissues is about 75 milligrams of amino-nitrogen per hundred 

 grams of tissue. This characteristic limit cannot be overstepped and 

 if the quantity of amino-nitrogen brought to the tissues exceeds their 

 Assimilation-limit the excess of amino-acids in the blood is destroyed 

 by Deaminization, the nitrogen being split off in the form of ammonia 

 which is converted by the liver into Urea. The Liver plays a leading 

 part in this process and continually and rapidly desaturates itself by 

 deaminization of the free amino-acids which it contains; doubtless 

 other tissues share this ability, but the power of the liver to deaminize 

 amino-acids is certainly in excess of that of other tissues, because, 

 although other tissues do not show any greater avidity than the liver 

 for amino-acids, and do not reach a higher saturation-limit of amino- 

 nitrogen, yet within a few hours after the saturation of all the tissues 

 with amino-acids which succeeds a protein-rich meal, or injection of 

 amino-acid mixture, the other organs all contain more amino-acid 

 material than the liver. When we consider, also, that the liver is an 

 exceedingly bulky organ, its possession of a high deaminizing power 

 ensures its overwhelming predominance in this process. 



When, for any reason such as that afforded by degenerative changes, 

 the deaminizing power of the liver is impaired, this mechanism for dis- 

 posing of undue excess of amino-acids may prove insufficient and the 

 kidneys may assist by excreting unaltered amino-acids. If, at the same 

 time, the introduction of amino-acids into the blood is unnaturally 

 rapid, as for instance by rapid injection of large amounts of amino-acids 

 in solution, the mechanisms for their disposal may prove to be entirely 

 inadequate and serious symptoms of intoxication, or even death may 

 ensue. 



The absorption of amino-acids from the blood is never complete, 

 and free amino-acids are still present in the blood even when the 

 amino-acid concentration in the tissues is far below the saturation- 

 limit. Evidently, therefore, we are dealing here with an equilibrium, 

 somewhat resembling the partition of a dissolved substance between 

 two immiscible solvents, increase in the amino-acid content of the blood 

 leading to increase, up to the saturation-limit, of the amino-acid con- 

 tent of the tissues, while the loss of tissue-protein which occurs in 

 Starvation indicates that diminution of the amino-acid content of the 

 blood may also lead to desaturation of the tissues by the passage of 

 amino-acids into the blood. The same mechanism, also, permits trans- 

 fer of particular amino-acids from one tissue to another and explains 

 the otherwise surprising fact that certain tissues, for example malignant 

 growths, may grow at the expense of other tissues, and also, in part, 

 the fact that the loss of weight of the various organs and tissues in 

 starvation is very unequal, certain tissues losing very heavily while 

 others retain their weight very nearly undiminished until death ter- 



