VARIATIONS IN THE ACIDITY OF THE URINE. 295 
2. But the kidneys have the special function of regu- 
5. 
lating the degree of alkalescence of the blood. 
When it is too high they separate alkali, and the 
urine becomes alkaline ; when it tends to become 
too low, on the other hand, they separate acid, 
and this gives to the urine its common acid re- 
action. 
A meal then, in so far as its mineral ingredients 
are concerned, is but a dose of alkali, and its ab- 
sorption causes, like any other dose of alkali, a 
depression of the acidity of the urine. 
. The emission of urine turbid with phosphates is, 
within certain limits, a natural phenomenon ; and ; 
earthy phosphates constitute the only urinary de- 
posit which can appear in the healthy urine on 
passing. 
Urines may be divided into two chief classes : — 
First, urines of fasting (urine sanguinis) ; secondly, 
urines of food (urine cibi). Fasting urines are 
scanty and of high acidity; they present only one 
variety, namely, that of sleep, which differs from 
other fasting urines in possessiug more colouring 
matter. The urines of food fall naturally into two 
divisions — those with a diminished, and those with 
a restored acidity; they are abundant in quantity. 
The urines of either class may be concentrated or 
dilute, according to the relation between potation 
and the requirements of the system in regard to 
water. So that urine potis do not merit to be 
regarded as a distinct class. Most commonly the 
urine of micturition belongs exclusively to no divi- 
sion, but is a mixture of several kinds. 
