Nutrition. 227 



The nitrogen is almost wholly excreted by the kidneys ; 

 a small portion may possibly be got rid of by the lungs, and 

 in working horses by the skin. It is usual to regard the 

 urine nitrogen as a measure of the proteid changes in the 

 system, and this nitrogen is got rid of in the form of urea, 

 hippuric acid, colouring matter, and probably ammonia. 



The sulphur of the body is got rid of through the kidneys, 

 and by means of the cast-off epithelium, hair, and horn. 



It is now necessary to glance at the form in which the 

 various elements of the body enter it, and we will deal first 

 with proteids. 



Nitrogenous Food. — If an animal be fed exclusively on a 

 proteid diet, it is found that, practically, the more it gets 

 the more nitrogen is excreted by the kidneys, until at 

 length a stage arrives when the amount of proteid excreted 

 equals the amount taken in; this condition does not list 

 long, and the term applied to it is 'nitrogenous equilibrium.' 

 These experiments have been made on dogs, and it has 

 been observed that in order to produce this nitrogenous 

 equilibrium, a considerable quantity of flesh has had to be 

 ingested, the result being that the animal has gained weight. 

 It is evident, therefore, that if the amount of nitrogen 

 passing out of the body is equal to the amount passing in, 

 and yet, in spite of this fact, the animal is gaining weight, 

 a something in proteid food which is not nitrogen is being 

 stored up ; this something is the carbon portion of the 

 proteid substance, and it is found that this is stored up in 

 the body in the form of fat. From these observations it 

 has been determined that a proteid breaks up in the body 

 into two portions : one, the nitrogenous portion, which is 

 excreted as urea ; the other, the non-nitrogenous portion, 

 which is stored up in the system as fat. 



Under ordinary circumstances the whole of the nitrogen 

 of the proteid is not excreted as urea ; a portion remains in 

 the system and is converted into tissue. According to 

 some observers the nitrogenous portion which gives rise to 

 urea has been termed ' the circulating albumin,' the other 

 or smaller portion 'the tissue albumin.' 



15—2 



