262 LECTURE XII. 



relations somewhat more in detail, because they have hitherto received 

 but little attention in experimental investigation. 



The albuminous substances now produced, together with the ammonia 

 that has been split off, and, possibly, other cleavage-products from the 

 amino acids, pass from the intestine to the liver, and from here into the 

 general circulation. We will mention that the liver is quite generally 

 considered as the place where the above-mentioned aromatic products of 

 putrefaction are conjugated with sulphuric and glucuronic acids. We 

 have already mentioned the fact that urea is formed in the liver on a large 

 scale. 



As a consequence of the extensive decomposition and reconstruction of 

 proteins in the alimentary tract, only those albuminous substances circu- 

 late in the organism which correspond to its entire construction. Every 

 cell continually receives the same nourishment in the same composition, 

 through the instrumentality of the blood. The whole mechanism of 

 the cell is thereby greatly simplified. The cell is, in the widest sense, 

 independent of the nature of the food in the exercise of its functions. 

 It is, in its entire metabolism, adjusted to a nourishment of quite definite 

 composition, which is always available to satisfy its demands. A prom- 

 inent part is taken by the intestine in the total metabolism. The 

 nutritional relations of the entire cell-material depend in the widest sense 

 upon its activity. Its functions are simplified, in proportion as the 

 proteins from the food are prepared by the combined action of hydro- 

 chloric acid and pepsin and of trypsin. The cells of the intestines will 

 be built up more quickly the better the material available for synthesis 

 is suited for the new proteins. Although any derangement in the 

 secretion of the ferments would undoubtedly affect the processes in the 

 intestinal tract, the disturbance in this tract must affect the entire meta- 

 bolism even more seriously. 



The individual body-cell removes from the blood such protein sub- 

 stances as it requires. It breaks them down in the same manner that 

 trypsin does. Amino acids result, which, on further decomposition, 

 produce, on the one hand, urea, and, on the other, carbon chains free 

 from nitrogen, of whose nature we still know but little, and which, 

 perhaps, enter into relations with the carbohydrates, fats, and, possibly, 

 other organic components of the tissues. The breaking down of the 

 amino acids in the cells is still an obscure process. We only know that 

 the total nitrogen soon appears in the urine. It is questionable whether 

 the total combustion of the amino acids is immediately effected after the 

 elimination of the amino or CO . NH 2 groups, or whether the surviving 

 carbon chains in their further decomposition are independent of the above 

 process. The formation of the nitrogenous end-products, whether it be 

 urea or uric acid, is also but imperfectly explained. If we assume that the 



