782 SPECIAL PHYSIOLOGY. 



depend upon free phosphoric acid, or, an acid phosphate of soda, or 

 perhaps upon this acid, together with a minute quantity of dissolved 

 uric acid ; for it is difficult to suppose the existence of free lactic acid, 

 so long as any alkaline urates are present, and these latter salts may 

 always be obtained by the quick evaporation, in vacua, of perfectly 

 fresh urine. The acidity of the urine gradually diminishes for from 

 three to five hours after a meal, and sometimes the excretion becomes 

 actually alkaline. This effect occurs simultaneously with the develop- 

 ment of the large quantity of acid in the gastric juice poured out for 

 the digestion of the food ; whilst the return of the urine to its acid condi- 

 tion during fasting, corresponds with the cessation of the formation of 

 acid in the stomach. This temporary diminution in the acidity of the 

 urine, or its positive alkalinity, is most marked when animal food is 

 taken, which requires more acid to digest it; with vegetable food it is 

 less so ; with mixed diet, the effects are intermediate. (Roberts.) The 

 alkalinity of the urine after a vegetable diet, and of that of the Her- 

 bivora generally, is not opposed to these observations, and has another 

 explanation. In such diet, large quantities of neutral alkaline salts 

 of the vegetable acids are met with, which are converted in the ali- 

 mentary canal, or in the blood, into carbonates ; the quantity of albu- 

 minoid food or tissue metamorphosed is so small as not to yield enough 

 sulphuric and phosphoric acid to neutralize this alkali. The urine is 

 also often alkaline in gastric disorder. In the Carnivora, ammonia 

 acts the part of a base to the acids of the urine, instead of the fixed 

 alkalies. 



Oxalic acid, C 2 3 -f H 2 0, also occurs in the urine, especially after 

 eating fruit, which contains organic acids, also after drinking fluids 

 containing free carbonic acid, and, lastly, when the respiratory pro- 

 cess is seriously disturbed. Any condition which tends to overload 

 the blood with carbonic acid favors the appearance of oxalic acid in 

 the urine ; in children it is a frequent constituent, and, in combination 

 with lime, forms the comparatively common mulberry calculus. Lastly, 

 carbonic acid itself is found in a state of solution, in the quite recently 

 discharged urine of both man and animals. Besides this, the urine con- 

 tains nitrogen, with traces of oxygen, and, but only as a product of 

 decomposition, sulphuretted hydrogen. 



Minute traces of certain fats, such as olein and stearin, occur in 

 urine. In certain cases, a fatty substance, probably a mixture of 

 ordinary fats, named keistin, appears as a scum upon it; and, in 

 altered conditions of the kidneys, large quantities of oily matter rise 

 up to the surface. 



The salts found in the urine average about 1.8 per cent, of that 

 fluid, though they vary extremely, according to the character of the 

 food and the amount of fluid ingesta ; the latter increase their quan- 

 tity. Of 100 parts of these salts, the sulphates form 45 parts, the 

 phosphates 24, and the chlorides 23, the residue consisting of salts of 

 the organic acids. The chief base is soda, next in order potash, then 

 ammonia, magnesia, lime, and lastly, in minute quantities, iron and 

 silica. Whilst most of these salts are derived directly from the mate- 

 rials of the food, others undoubtedly proceed from the metamorphoses 



