36 PHYSIOLOGICAL CHEMISTRY. 



that they assume, for the time being, a fluid form. Water is therefore 

 an essential ingredient of the animal fluids, for it holds their ingredients 

 in solution, and enables them to pass and repass through the animal 

 frame. 



But water is a constituent also of the solids. If a muscle or a carti- 

 lage be exposed to gentle heat in dry air, it loses water by evaporation, 

 diminishes in bulk, and becomes dense and stiff. Even the bones and 

 teeth lose water in this way, though in smaller quantity. In all the 

 solid and semi-solid tissues, the water which they contain is useful by 

 giving them the special consistency which is characteristic of them, and 

 which would be lost without it. Thus a tendon, in its natural condi- 

 tion, is white, glistening, and opaque ; and, though very strong, per- 

 fectly flexible. If its water be expelled by evaporation it becomes 

 yellowish, shrivelled, semi-transparent, inflexible, and unfit for perform- 

 ing its mechanical functions. The same is true of the skin, the muscles, 

 the cartilages, and the glands. 



The following is a list, compiled by Robin and Verdeil from various 

 observers, showing the proportion of water per thousand parts in dif- 

 ferent solids and fluids : 



QUANTITY OF WATER IN 1000 PARTS IN 



Teeth .... 100 Bile .... 880 



Bones .... 130 Milk . . . .887 



Cartilage. . . . 550 Pancreatic juice . . 900 



Muscles . . . .750 Urine . . . .936 



Ligaments . . .768 Lymph . . . .960 



Brain . . . .789 Gastric juice . . .975 



Blood . . . .795 Perspiration . . .986 



Synovial fluid . . . 805 Saliva . . . .995 



According to the best calculations, water constitutes, in the human 

 subject, about seventy per cent, of the entire bodily weight. 



The water which thus forms part of the animal frame is derived 

 mainly from without. It is taken in the form of drink, and is also 

 abundant in various kinds of food. For no articles of food are taken in 

 an absolutely dry state, but all contain more or less water, which may 

 be expelled by evaporation. The quantity of water, therefore, daily 

 taken into the system, cannot be ascertained by simply measuring the 

 quantity of drink, but its proportion in the solid food must also be 

 determined, and this quantity added to that taken in with the fluids. 

 By measuring the fluid taken as drink, and calculating in addition its 

 proportion in the solid food, we have found, in accordance with the 

 results formerly obtained by Barral, that, for a healthy adult man, the 

 average quantity of water introduced into the system is about 2000 

 grammes per day. 



There is reason to believe that a certain quantity of water also makes 

 its appearance within the body by the liberation of its elements from 

 various organic combinations. This is shown by the fact that a con- 

 siderable quantity of hydrogen is daily introduced into the system in 



