126 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



A certain part of the water found in animal tissues is held in com- 

 bination, as in water of crystallization, both in organic and inorganic 

 molecules. This amount is, however, inconsiderable as contrasted with 

 that held in other manners. Water is also found as a vapor in the air 

 contained in the respiratory organs of animals. 



By far the greater part of the water found in the animal and vege- 

 table body has entered from without ; in the former case through the 

 food and drink, and in the latter from rain or from the absorption of 

 moisture from the soil. In the case of the animal body a certain amount 

 of water is apparently formed within the animal economy, since it has 

 been found that under certain circumstances the amount of watery vapor 

 exhaled through the lungs and skin,, and that passing through the kid- 

 neys and intestines, is in excess of the amount of water taken internally, 

 the body still preserving its uniform weight. Again, as we shall find in 

 considering the subject of respiration, the volume of carbon dioxide 

 eliminated through the lungs is smaller than the amount of oxygen taken 

 into the blood in inspiration. Ten to twenty-five per cent, of oxygen 

 disappears in this manner, and must, therefore, have formed other com- 

 binations in the body than those whose end product is C0 3 . Since it is 

 readily conceivable that the hydrogen of hydrogen compounds is set free 

 quite as readily as the carbon of carbon compounds, a certain amount 

 of this hydrogen may evidently unite with oxygen to form water, not by 

 a direct oxidation of the hydrogen, but through the gradual union of 

 the oxygen with a long series of oxidation products whose terminal is 

 H 2 0, just as C0 2 results from the final union of oxygen and carbon, and 

 not by a direct oxidation of carbon in the animal body. Such an origin 

 of water in the economy is further supported by the fact that the amount 

 of hydrogen contained in organic compounds in the excretions is less 

 than that which is contained in similar combinations in the food. Thus, 

 it has been estimated that a man receives daily forty grammes of hydrogen 

 in organic combinations with the food, while only six grammes are dis- 

 charged in such combinations in the excretions ; hence, thirty-four 

 grammes, or about 85 per cent, of the hydrogen so introduced, remains 

 unaccounted for. Since hydrogen does not leave the body as a vapor, 

 nor in any notable amount in any other inorganic compound but water, 

 the surplus must be converted into water. The estimates are that in 

 man about three hundred grammes of water are formed daily in this 

 wa Y — probably from the decomposition of carbo-hydrates where hydro- 

 gen and oxygen are present in the proportion to form water. 



Organisms not only live in water, but they may be said to live in 

 running water (Hoppe-Seyler) ; for they are continually taking in water, 

 which may contain other food-stuffs in solutions, and are continually 

 eliminating water which contains the products of their tissue-waste. 



