124 PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



as constituents of tissues perhaps in the solid or even crystalline form, 

 but are again eliminated from the body in solution ; this applies not only 

 to the salts, but also to the gases and acids. Many of the physical prop- 

 erties of various tissues depend almost solely upon their inorganic 

 constituents. In this connection it is only necessary to mention the 

 bones and teeth. "Wherever cell growth is taking place certain salts are 

 essential, since no form of protoplasm is able to carry on its existence 

 without a supply of salts, the nature of which may differ in the case of 

 different cell forms ; thus, for example, calcium salts are not only 

 essential for the development of the bone-cells, but accompany the 

 albuminoids of all growing tissue ; blood-corpuscles require iron and 

 potassium phosphate, and all forms of cell growth require sodium 

 chloride. 



1. Water (H s O). — Of the inorganic constituents of cells water is by 

 far the most abundant, and is the most important. In fact, all organisms 

 may be said to live in water ; for if their entire body is not surrounded by 

 water, all contain water in large amounts, and all their vital processes are 

 dependent on watery solutions. Water is destined, by entering by imbi- 

 bition into solid tissues, not only to preserve the physical condition 

 which is essential to the preservation and manifestations of the vital 

 phenomena of protoplasm, but it holds in solution many of the salts 

 essential to the vital processes of the economy. It also constitutes a 

 large proportion of the fluids of the body, such as the blood, lymph, chyle, 

 and secretions. It is in greater amount in embryonic tissue, and decreases 

 as adult life and old age are reached. In the higher animals it may 

 exist in 70 per cent, or more, while in some of the lower forms of life as 

 much as 90 per cent, may be reached. The amount of water in different 

 organisms, and in the same organism at different times, is subject to 

 very great variation. It not only constitutes the great part of the secre- 

 tions of the animal body, but it also forms a large proportion of even 

 the densest tissues of the animal or vegetable body. Thus, in the 

 enamel of teeth two-tenths of one per cent, of water is present, while in 

 dentine 10 per cent, and in bones 22 per cent, of water is found. 



The following table represents the amount of water in 1000 parts of 

 different animal tissues : — 



Organs. 



Enamel, . ... 



Ivory, . ... 



Bone, . 



Pat, . 



Efastic tissue, 



Cartilage, . 



Liver, 



Bone-marrow, . 



White brain-substance, 



Water. 



Solid 



2 



998 



100 



900 



216 



784 



209 



701 



40(i 



504 



550 



450 



693 



307 



097 



303 



700 



300 



