THE KIDNEYS. SECRETION OP URINE. 607 



quantity gives it importance is Chloride of Sodium. By far the larger pro- 

 portion of this is doubtless derived directly from the food; but little being 

 furnished by the disintegration of Muscle, which will set free potash rather than 

 soda ( 307). The amount eliminated by the urine is consequently subject to 

 great variation, it being the function of the Kidneys to remove whatever is 

 superfluous, so as to prevent the blood from becoming overcharged with this 

 substance ( 82). Of the chloride of sodium introduced as food, a part appears 

 to undergo decomposition in the system, whereby hydrochloric acid is furnished 

 to the gastric fluid, and soda to the bile ; part of this acid, however, must re- 

 unite with its base in the alimentary canal, so that the chloride of sodium thus 

 regenerated will be absorbed with the products of the digestive operation. 

 Although Nitric Acid cannot be regarded as a normal constituent of the Urine, 

 yet the recent investigations of Dr. Bence Jones 1 show that it is formed by a com- 

 bustive process within the body, whenever ammoniacal salts are introduced into 

 the system ; its amount, however, being very small. He has also found that it 

 is generated after the ingestion of small quantities of urea; a fact which affords 

 some confirmation to the doctrine of Frerichs ( 637), that urea may undergo 

 decomposition into carbonate of ammonia, whilst still circulating in the current 

 of blood. The presence of Oxalic Acid in the urine (in combination with Lime) 

 has been usually regarded as a pathological phenomenon, consequent upon an 

 irregular performance of the retrograde metamorphosis of the tissues ; but 

 there can be no doubt that it may also result from the presence of soluble salts 

 of oxalic acid in certain articles of vegetable food. 3 



642. 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, which 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 or- 

 ganic acids are almost certainly present. 3 It has been shown by the researches of 

 Dr. Bence Jones, however, 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 with 

 the acidity of the stomach ( 443). Thus the acidity of the Urine decreases 

 soon after taking food, whilst that of the Stomach is increasing ; and attains 

 its lowest limit from three to five hours after a meal, frequently giving place to 

 an alkaline reaction. 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 its acidity was never such as to render the urine absolutely alka- 

 line, 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 probable that corresponding variations occur 

 in the alkalescence of the blood ; such diurnal variations being produced by the 



1 "Philosophical Transactions," 1851. 



2 See Dr. Golding Bird on "Urinary Deposits," 2d Am. Ed., p. 188. 



3 See Prof. Lehmann's "Lehrbuch der Physiologischen Chemie," band ii.pp. 398-400. 



