22 HUMAN PHYSIOLOGY. 



substance which gelatinizes on cooling, and to which the name 

 chondrin has been given. 



(c) Elastin is the name given to the substance composing the fibers 

 of the yellow, elastic connective tissue. 



(d) Keratin is the substance found in all horny and epidermic tissues, 

 such as hairs, nails, scales, etc. It differs from most proteins in 

 containing a high percentage of sulphur. 



INORGANIC CONSTITUENTS. 



The inorganic compounds and mineral constituents obtained from the 

 solids and fluids of the body are very numerous, and, in some instances, 

 quite abundant. Though many of the compounds thus obtained are 

 undoubtedly derivatives of the tissues and necessary to their physical and 

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

 or transitory constituents introduced with the food. Of the inorganic com- 

 pounds, 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 chem- 

 ically 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 



