406 EXCRETION 



time exclusively upon animal substances, presents the acid reaction and 

 other qualities of the urine of carnivora, and its ordinary alkalinity is again 

 restored only on the substitution of a vegetable for the animal diet. Human 

 urine is not usually rendered alkaline by vegetable diet, but it becomes so 

 after the free use of alkaline medicines or of the alkaline salts with carbonic 

 or vegetable acids. These latter are changed into alkaline carbonates 

 previous to elimination by the kidneys. 



Specific Gravity of Urine. The average specific gravity of the human 

 urine is about i .020 to i .025. The relative quantity of water and of solid 

 constituents of which it is composed is materially influenced by the condition 

 and occupation of the body during the time at which it is secreted; by the 

 length of time which has elapsed since the last meal; by the amount of water 

 taken; and by several other less important circumstances. The morning 

 urine is the best adapted for analysis in health, since it represents the simple 

 secretion unmixed with the elements of food or drink. If it is not used the 

 whole of the urine passed during the period of twenty-four hours should be 

 taken. The specific gravity of the urine may thus, consistently with health, 

 range widely on both sides of the usual average. It may vary from 1.015 

 in the winter to 1.025 in the summer; but variations of diet and exercise, 

 and many other circumstances, may make even greater differences than 

 these. The variations may be extreme in disease, sometimes decreasing in 

 chronic nephritis to i . 004, and frequently increasing in diabetes, when the 

 urine is loaded with sugar, to i .050 or even to i .060. 



AVERAGE DAILY QUANTITY OF THE CHIEF URINARY CONSTITUENTS (MODI- 

 FIED FROM PARKES.) 



Variations in the Constituents of Urine. Most of the constituents 

 are, even in health, liable to variations from the proportions given in the 

 above table. The variations of the quantity of water in different seasons 



