WATER REGARDED AS A PRODUCT OF EXCRETION. 389 



These proportions represent the average of fifteen observations upon the 

 urine secreted during the night. 



The proportion of these gases was found by Morin to be subject to certain 

 variations. For example, after the ingestion of a considerable quantity of 

 water or any other liquid, the proportion of oxygen was considerably increased 

 (from O824 to 1-024), and the carbon dioxide was diminished more than one- 

 half. The most important 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 carbon dioxide was found to be 

 double that contained in the urine after repose. The proportion of oxygen 

 was very slightly diminished, and the nitrogen was somewhat increased ; but 

 the variations of these gases were insignificant. 



It is not probable that the kidneys are very important as eliminators of 

 carbon dioxide, but it is certain that the presence of this gas in the urine 

 assists in the solution of some of the saline constituents of this fluid, notably 

 the phosphates. 



Water regarded as a Product of Excretion. It has been shown by indi- 

 rect observations that a large proportion of the hydrogen introduced as an 

 ingredient of food, about eighty-five per cent., is not accounted for by the 

 hydrogen of the excreta. Direct observations have shown, also, that under 

 certain conditions, an excess of water over that introduced with food and 

 drink is discharged from the body. -One of these conditions is abstinence 

 from food (Flint, 1878). The elimination of water is very much increased 

 by muscular work (Pettenkofer and Voit, 1868 ; Flint, 1879). These facts 

 point to the actual production of water in the body by a union of oxygen 

 with hydrogen. 



While it is not certain that water is constantly produced in the body, 

 there can be no doubt with regard to its formation under some conditions, 

 and the oxidation of hydrogen is important as one of the factors in the pro- 

 duction of animal heat. If a certain proportion of the water discharged by 

 the lungs, skin and kidneys be regarded as a product of oxidation within the 

 body, the relations which it bears to nutrition are probably the same as those 

 of some of the excretions, especially carbon dioxide, and are subject to nearly 

 the same laws. It has not been shown, however, that water is produced 

 constantly, like those substances universally regarded as true excretions ; and 

 it gives rise to no direct toxic phenomena when retained in the system or 

 when its production is diminished pathologically. Water also has important 

 physiological uses, particularly as a solvent. Still, carbon dioxide, with 

 which water may be compared as regards its mode of production, is not in 

 itself poisonous, its retention in the blood simply interfering with the absorp- 

 tion of oxygen ; and carbon dioxide probably is useful in increasing the solvent 

 properties of the liquids of the organism. The relations of the formation of 

 water in the body to the production of animal heat will be fully considered 

 in connection with the physiology of nutrition and calorification. 



Variations in the Composition of the Urine. The urine not only repre- 

 sents, in its varied constituents, a great part of the physiological disintegra- 



