516 



OP SECRETION AND EXCRETION. 



About one-third of the carbonic acid was combined, the remaining two- 

 thirds were free. 



415. The ordinary acid reaction of the Urine appears to be due, not to 

 the presence of any free acid, but to the conversion of the basic phosphate 

 of soda into the acid phosphate, by the subtraction of a part of the base, 

 Avltich occurs when uric, hippuric, lactic, or other free acids come into contact 

 with the former substance. There is no adequate reason to believe, that, in 

 the healthy state, there is ever any other cause than this; although in morbid 

 urine, free organic acids are almost certainly present. 1 It has been shown 

 by the researches of Dr. Bence Jones, 2 with whose observations those of 

 Roberts 3 and Bruuton 4 generally agree, that the acid reaction is far from 

 being constant in its degree, even when an ordinary mixed diet is steadily 

 employed ; for that it varies at different periods of the day, increasing and 

 decreasing inversely ivith the acidity of the stomach. Thus the acidity of the 

 Urine decreases soon after taking food, whilst that of the Stomach is increas- 

 ing, and attains its lowest limit about three to five hours after a meal, when, 

 therefore, absorption is being most actively performed. An alkaline reaction 

 is often observed in the urine at this period, which, according to Dr. Roberts, 

 is due to the presence of fixed alkali ; this occasions a precipitation of earthy 

 phosphates, rendering the urine turbid, though the quantity of uric acid at 

 this period is always large. The acidity then gradually increases, whilst 

 that of the stomach is decreasing; and attains its highest limit after a fast 

 of some hours, when the stomach is quite empty, and its secretion neutral. 

 If no food be taken, the acidity does not decrease, but remains at nearly the 

 same point for ten or twelve hours. When animal food was alone employed, 

 the diminution of the acidity after a meal was more marked and continued 

 longer than when a mixed diet was eaten (apparently on account of the 

 greater demand for acid in the stomach) ; and the acidity did not rise quite so 

 high after fasting, as with a mixed diet. On the other hand, when the diet 

 was purely vegetable, the diminution of the acidity of the urine was never 

 such as to render it absolutely alkaline, although its acidity was reduced to 

 the point of neutrality ; and the increase of its acidity after fasting was 

 sometimes very considerable, though by no means so marked as the decrease 

 of alkalescence. These diurnal variations in the acidity of the urine make 

 it highly pro-bable, that corresponding variations occur in the alkalescence 

 of the blood ; such diurnal variations being produced by the quantity of acid 

 separated from it, and poured into the stomach for the purpose of dissolving 

 the food. The introduction of dilute sulphuric acid into the stomach, even. 



1 See Prof. Lehmann's Physiological Chemistry (Cavendish Society's ed.), vol. ii, 

 pp. 404-400. 



2 Philos. Transact., 1849. 



3 Memoirs of Manchester Phil. Society, 1859. 4 Digitalis, p. 62. 



