CHEMIC COMPOSITION OF THE HUMAN BODY. 25 



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. Posses- 

 sing the capability of holding in solution a large number of inor- 

 ganic 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 three 

 kilograms (6^ pounds). 



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 extent 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 can not be separated 

 from it except by chemic means, such as hydrochloric acid. Though 

 insoluble in water, it is held in solution in the blood and milk by the 

 proteid constituents, and in the urine by the acid phosphate of soda. 

 The total quantity of calcium phosphate which enters into the for- 

 mation 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 situ- 

 ations in the body as the phosphate, and plays essentially the same 

 role. It is, however, found in the crystalline form, aggregated in 

 small masses in the internal ear, forming the otoliths, or ear stones. 



