WATER. 41 



pen sable in order to give them the fluidity which is necessary to the 

 performance of their functions ; for it is by the blood and secretions 

 that new substances are introduced into the body, and old ingredients 

 discharged. And it is a necessary condition both of the introduction 

 and discharge of substances naturally solid, that they assume, for the 

 time being, a fluid form ; water is therefore an essential ingredient of 

 the fluids, for it holds their solid materials in solution, and enables them 

 to pass and repass through the animal frame. 



But water is an ingredient also of the solids. For if we take a muscle 

 or a cartilage, and expose it to a gentle heat in dry air, it loses water 

 by evaporation, diminishes in size and weight, and becomes dense and 

 stiff. Even the bones and teeth lose water by evaporation in this way, 

 though in smaller quantity. In all these solid and semi-solid tissues, 

 the water which they contain is useful by giving them the special con- 

 sistency which is characteristic of them, and which would be lost without 

 it. Thus a tendon, in its natural condition, is white, glistening, and 

 opaque ; and though very strong, perfectly flexible. If its water be 

 expelled by evaporation it becomes yellowish in color, shrivelled, semi- 

 transparent, inflexible, and unfit for performing its mechanical functions. 

 The same thing is true of the other tissues, such as that of the skin, the 

 muscles, the cartilages, and the glands. 



The following is a list, compiled by Robin and Yerdeil 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 weight of the body. 



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

 mainly from without. It is taken in the different kinds of drink, and 

 also forms an abundant ingredient in the various articles of food. For 

 no articles of food are taken in an absolutely dry state, but all contain 

 a larger or smaller quantity of water, which may readily be expelled by 

 evaporation. The quantity of water, therefore, which is daily taken into 

 the system, cannot be ascertained in any case by simply measuring the 

 quantity of drink, but its proportion in the solid food, taken at the same 

 -time, must also be determined by experiment, and this ascertained 

 quantity added to that which is taken in with the fluids. By measuring 

 the quantity of fluid taken with the drink, and calculating in addition 

 4 



