The Urine. 209 



In sheep urea and hippuric acid stand in the proportion 

 of 2 to 3, whereas in cattle on the same diet the proportion 

 is 16 or 20 of urea to 11 or 13 of hippuric acid. 



It is strange that the food most productive of hippuric 

 acid in the horse is the least productive of this substance 

 in the sheep. Old meadow hay produces a large quantity 

 of hippuric acid in the horse, whilst new meadow hay has 

 this effect on sheep. 



In sheep there is three times more magnesia than lime 

 in the urine, whereas the reverse is the case in the faeces of 

 the same animal. 



The urine of the pig resembles that of carnivora, but its 

 composition, etc., depends on the character of the food. 

 The specific gravity is 1010 to 1015, it is either acid or 

 alkaline, and contains uric acid, xanthin, guanin, and much 

 urea. 



Secretion of Urine. — The physiology of the secretion of 

 urine was touched on at the early part of this chapter 

 (p. 197) ; it is a comparatively simple process, and consists 

 in the difference in pressure between the blood in the 

 Malpighian bodies and the tubules; in this it differs 

 markedl} 7 from the saliva, gastric juice, and bile (Landois 

 and Stirling). When the pressure in the tubules amounts 

 to two-thirds of the pressure in the renal arteries, the 

 secretion of urine ceases. 



This theory of urinary secretion has been termed the 

 filtration theory, but there are certain facts which tend to 

 show that such a view of the matter is incomplete, and that 

 it is necessary to take into consideration the vital activity 

 of the cells lining the tubules. Blood pressure doubtless 

 plays an important part in the process, but it will not 

 account for an acid urine, nor the presence of a large 

 quantity of such substances as urea, hippuric acid, creatine, 

 etc., which are either absent from the blood or only found 

 in it in traces, and these are beyond doubt excreted by the 

 epithelial cells of the tubules. 



We have before drawn attention to the parts of the 

 kidney in which the various constituents are excreted. 



14 



