160 GENERAL PHYSIOLOGY 



9 



which the plant receives through its roots from nitrates and 

 sulphates only, complicated transformations of these salts and 

 then syntheses with the carbohydrate atoms must take place, 

 the details of which are thus far wholly unknown. As to how, 

 finally, the proteid molecule, synthetically formed, is employed 

 further in the living substance for purposes of construction, at 

 present, on account of our extremely scanty knowledge of the 

 chemical constitution of proteids, we can say absolutely nothing. 

 Here an enormous field is offered for future physiological investi- 

 gation. 



In animals, the path from the ingested food to the living 

 proteid molecule is of course essentially shorter, for all animals 

 without exception need for their nutrition proteids already pre- 

 pared. But what happens further to the proteids that have been 

 peptonised by digestion is not fully known. After the investigations 

 of Salvioli ('80), Hofmeister ('82), Neumeister ('90), and others, 

 no doubt can be entertained that the peptones as such disappear 

 in the cells of the wall of the intestine, in other words, they 

 are transformed in the cell itself. If pieces of the intestinal mucous 

 membrane of a rabbit be placed in a liquid that contains peptone, 

 in which the cells of the intestinal wall exist during life, after 

 some time it is found that all peptone has disappeared. If, 

 moreover, a solution of peptone be injected into the blood of an 

 animal, in a short time the whole quantity of peptone is excreted 

 unchanged in the urine ; and in normal life the blood is always 

 free from peptones. These two experiments prove undoubtedly 

 that the peptones become changed on their way through the cells 

 of the intestinal wall. But little is thus far known as to the kind 

 of change within the cells. Perhaps some of the peptones are 

 broken down immediately into simpler substances by a retrogres- 

 sive proteid metamorphosis. It is certain that many are changed 

 back into proteid and pass into the juices of the body along with 

 the proteid resorbed directly without peptonisation. This 

 dissolved proteid circulates throughout the body with the blood- 

 current, bathes the cells of all tissues, and is withdrawn by the 

 cells from the blood, to be broken down within them. Hence it 

 happens that in a remarkably short time all the proteid taken into 

 the body, beyond a certain quantity, appears as urea, uric acid, 

 etc., in the urine. Voit ('81) thought that this proteid that is 

 broken down ought to be distinguished as " circulating proteid " 

 from the " tissue proteid/' which is employed for the formation of 

 tissues, since he assumed that the destruction of the circulating 

 proteid took place in the blood, in the liquids, of the body. But 

 the reason for such a distinction has disappeared, since Pfliiger 

 ('93) and Schondorff ('93) have shown recently by very careful 

 investigations that the breaking-down of the proteid dissolved in 

 the blood does not take place in the, blood itself, but in the tissue- 





