VARIATIONS IN THE ACIDITY OF THE URINE. 285 
Now, what does this increase in the solids of the urine 
after a meal indicate? It evidently marks the passage of 
digested food into the blood. We see reflected in the 
renal secretion, as in a mirror, the flow of aliment into the 
blood. First the small beginning, then the increase, then 
the full tide, followed by a gradual ebb as the last portions 
of the food continue to be taken up from the lower divi- 
sions of the alimentary canal; until at length, after all has 
been absorked, the blood returns to its condition as before 
a meal. 
No doubt the passage of food into the blood is not the 
only circumstance that causes an increased separation of 
solid urine. Active functional exercise, whether of muscle 
or brain, probably quickens the activity of the kidneys 
by throwing into the blood an increased quautity of effete 
materials — the products of the destructive metamorphosis 
of the muscular and nervous elements. The invariably 
great and sudden rise in the solid urine from eight to 
nine in the morning is certainly not entirely the effect 
of breakfast, as it is incredible that digestion and absorp- 
tion should have made such progress in forty minutes. 
The awakening of the whole system from the torpor of 
sleep— the aroused mental and muscular activity, and the 
consentaneous acceleration of the circulation and respira- 
zion — contributed, unquestionably, in producing the re- 
sult. No such sudden rise is seen at any time in the 
subsequent hours, nor after dinner.* 
The question now arises: How far is the depressing 
action of food on the acidity of the human urine a univer- 
sal phenomenon? It is impossible to solve this question 
* It may be noted that the increase in the hourly secretion of solid urine 
after dinner was not so prominently marked as after breakfast. I believe 
this arose from the almost complete inactivity usually observed by the 
subject of experiment for two or three hours after the former meal, whereas 
after breakfast his condition was one of activity. 
