324 A MANUAL OF VETERINARY PHYSIOLOGY 



and converted into urea and ornithine. No arginase is found in 

 muscle. 



The largest part of the urea excreted is not produced by the 

 metabolism of tissue protein, but from the products of food 

 absorption, and these are mainly represented by the ammonia 

 compounds taken up from the intestinal area. The urea con- 

 tribution effected by the tissues may also be represented by 

 ammonia compounds and monamino-acids, but probably more 

 especially by the splitting up of arginine. Uric acid has been 

 considered by some physiologists as a possible source of urea, 

 for it has been observed in man that about one-half of the 

 uric acid produced is not excreted as such, but is got rid of as 

 urea ; in dogs, however, only one-twentieth undergoes this con- 

 version. This change is effected by a mucolytic ferment found 

 in the liver and other organs. Uric acid given by the mouth 

 is excreted as urea, and outside the body it is readily converted 

 into urea by oxidation ; nevertheless, this is not very strong 

 evidence in favour of the existence of uric acid conversion in 

 the living body. Kreatinine was at one time regarded as a source 

 of urea, but with a better knowledge of this substance and 

 kreatin, the conversion of kreatinine into urea, which offers no 

 difficulty as a laboratory process, is regarded in the living 

 body as doubtful. 



There is one remaining source of urea to be glanced at, and 

 that is the urea which is supposed to be capable of manufacture 

 in the tissues. It has been found that when the liver is removed 

 urea does not disappear from the urine, and it is supposed that 

 the origin of this portion is from the tissues, though not neces- 

 sarily formed in the same way as the liver-urea, nor from the 

 same material. 



The proportion of urea in urine varies dependently on the 

 nature of the diet. In man and the dog the larger the amount 

 of nitrogen in the food the more urea excreted, but in herbivora 

 it has been observed that on a diet consisting principally of hay 

 more urea is excreted than on one of oats and hay. Urea was 

 at one time considered to be a measure of the amount of work 

 performed by the animal body, but this view has long been 

 known to be wrong, though there can be no doubt that under the 

 influence of work rather more urea may be excreted than during 

 rest. 



Apart from these marked causes of variation there are others, 

 which certainly in the case of the horse lead to great differences 

 in the daily excretion, even under identical conditions, of work, 

 food, and rest. We shall . see also that "this is so in the dog, 

 an animal in which the widest variations occur in the urea. 

 In the horse, about ioo grammes (3^ ounces) daily is a mean 



