20 TEXT-BOOK OF PHYSIOLOGY. 



physiologic activity, others, in all probability, are decomposition products, 

 or transitory constituents introduced with the food. Of the inorganic 

 compounds, the following are the most important: 



WATER. 



Water is the most important of the inorganic constituents, as it is in- 

 dispensable to life. It is present in all the tissues and fluids without excep- 

 tion, varying from 99 per cent, in the saliva to 80 per cent, in the blood, 75 

 per cent, in the muscles to 2 per cent, in the enamel of the teeth. The total 

 quantity contained in a body weighing 75 kilograms (165 pounds) is 52.5 

 kilograms (115 pounds). Much of the water exists in a free condition, and 

 forms the chief part of the fluids, giving to them their characteristic degree of 

 fluidity. Possessing the capability of holding in solution a large number of 

 inorganic as well as some organic compounds, and being at the same time 

 diffusible, it renders an interchange of materials between all portions of the 

 body possible. It aids in the absorption of new material into the blood and 

 tissues, and at the same time it transfers waste products from the tissues to 

 the blood, from which they are finally eliminated, along with the water in 

 which they are dissolved. A portion of the water is chemically combined 

 with other tissue constituents and gives to the tissues their characteristic 

 physical properties. The consistency, elasticity, and pliability are, to a 

 large extent, conditioned by the amount of water they contain. The total 

 quantity of water eliminated by the kidneys, lungs, and skin amounts to 

 about 3 kilograms (6J pounds) daily. 



CALCIUM COMPOUNDS. 



Calcium phosphate, Ca 3 (PO 4 ) 2 , has a very extensive distribution 

 throughout the body. It exists largely in the bones, teeth, and to a slight ex- 

 tent in cartilage, blood, and other tissues. Milk contains 0.27 per cent. 

 The solidity of the bones and teeth is almost entirely due to the presence of 

 this salt, and is, therefore, to be regarded as necessary to their structure. 

 It enters into chemic union with the organic matter, as shown by the fact 

 that it cannot be separated from it except by chemic means, such as immer- 

 sion in hydrochloric acid. Though insoluble in water, it is held in solution 

 in the blood and milk by the protein constituents, and in the urine by the 

 acid phosphate of soda. The total quantity of calcium phosphate which 

 enters into the formation of the body has been estimated at 2.5 kilograms. 

 The amount eliminated daily from the body has been estimated at 0.4 gm., 

 a fact which indicates that nutritive changes do not take place with much 

 rapidity in those tissues in which it is contained. 



Calcium carbonate, CaCO 3 , is present in practically the same situa- 

 tions in the body as the phosphate, and plays essentially the same r61e. It 

 is, however, found in the crystalline form, aggregated in small masses in the 

 internal ear, forming the otoliths, or ear stones. Though insoluble, it is held 

 in solution by the carbonic acid diffused through the fluids. 



Calcium fluorid, CaF 2 , is found in bones and teeth. 

 SODIUM COMPOUNDS. 



Sodium chlorid, NaCl, is present in all the tissues and fluids of the 

 body, but especially in the blood, 0.6 per cent., lymph, 0.5, and pancreatic 

 juice, 0.25 per cent. The entire quantity in the body has been estimated 



