VARIATIONS LST THE TTBIXE. 219 



than one-half. The most interesting variations, however, 

 were in connection with muscular exercise. After walking 

 a long distance, the exercise being taken both before and 

 after eating, the quantity of carbonic acid was found to be 

 double that contained in the urine after repose. The pro- 

 portion of oxygen was very slightly diminished, and the 

 nitrogen was somewhat increased. The variations of these 

 gases, however, were insignificant. 



florin explains the great increase in the proportion of 

 carbonic acid, by the greater respiratory activity during 

 exercise. It is well known, indeed, that muscular exercise 

 largely increases the proportion of carbonic acid in the blood 

 and the quantity eliminated by the lungs ; and as the car- 

 bonic acid of the urine is undoubtedly derived from the 

 blood, we should expect that the same conditions would in- 

 crease its proportion in this secretion. 



It is not probable that the kidneys are very important as 

 eliminators of carbonic acid from the system, but it is cer- 

 tain that the presence of this gas in the urine assists in the 

 solution of some of the saline constituents of this fluid, no- 

 tably the phosphates. 



Variations in the Composition of the Urine. 



The urine represents in its varied constituents not only 

 a great part of the physiological disintegration of the organ- 

 ism, but it contains elements evidently derived from the 

 food. Its constitution is varying with every different con- 

 dition of nutrition, with exercise, bodily and mental, with 

 sleep, age, sex, diet, respiratory activity, the quantity of cu- 

 taneous exhalation, and, indeed, with every condition that 

 affects any part of the system. There is no fluid in the body 

 that contains such a variety of principles, as a constant con- 

 dition, but in which the proportion of these principles is 

 so variable. It is for this reason that we have given in 

 the table of the composition of the urine the ordinary lim- 



