146 THE BODY AT WORK 



assume that the degradation of proteins will occur in regular 

 steps ; complex, partially oxidized, nitrogenous compounds 

 being formed first in the muscles, for example to be further 

 oxidized in the glands. The existence in all organs of nitro- 

 genous " extractives," which can be separated out when the 

 organ is subjected to chemical analysis, seems to justify the 

 search for stages ; but hitherto this search has been singularly 

 unsuccessful. Urea is the final product. It is not found in 

 muscle, nor, indeed, in any tissue other than the liver, which, 

 as already said, has the power of making it, even from salts of 

 ammonia. It is therefore clear that if proteins are destroyed 

 in muscle and other tissues, and if all urea is made by the 

 liver, the antecedents of urea must be carried from the muscles 

 to this organ. The substance which is most characteristic of 

 muscular metabolism is lactic acid. It is not impossible that 

 all the nitrogenous portion of the complex proteid molecule is 

 reduced to ammonia (NH 3 ), which may be regarded as the 

 simplest of all nitrogenous compounds, and that this, com- 

 bined with lactic acid (C 3 H 6 3 ) as lactate of ammonia 

 (NH 4 C 3 H 5 3 ), is carried by the general circulation to the liver, 

 where it is converted into urea. A considerable amount of 

 lactate of ammonia may be injected into a vein without any 

 of it overflowing through the kidneys. It is all reduced to the 

 condition of urea, water, and carbonic acid. If the liver is so 

 diseased as to be functionless, or if by operative measures 

 it is thrown out of action, salts of ammonia are excreted by 

 the kidneys instead of urea. In birds and reptiles uric acid 

 takes the place of urea. Their livers yield uric acid on analysis. 

 If lactate of ammonia be injected into their blood, it is con- 

 verted into uric acid, so long as the liver is intact. 



We know nothing of the forms assumed by the proteins 

 absorbed into the blood, of the organs in which they are stored, 

 or of the higher terms of the series of substances through 

 which they pass before they are finally excreted as urea, 

 water, and carbonic acid. No nitrogenous compounds are 

 found in lymph or blood which can be pointed out with confi- 

 dence as the products of tissue wear and tear. When consider- 

 ing the sources of muscular energy, we shall have something 

 more to say regarding the part that proteins play in the economy. 



If there is great difficulty in following fats and proteins 



