22 TEXT-BOOK OF PHYSIOLOGY 



very small amount of this salt, but, as previously stated, a relatively large 

 amount of sodium chlorid. Though introduced to some extent in the food, 

 it is very likely that it is also formed through the decomposition of the so- 

 dium chlorid. 



Potassium phosphate, K 2 HPO 4 , is found in association with sodium 

 phosphate in all the fluids and solids. As it has similar chemic properties, 

 its functions are practically the same. 



Potassium carbonate, K 2 CO 3 , is generally found with the preceding 

 salt. 



MAGNESIUM COMPOUNDS. 



Magnesium phosphate, Mg 3 (PO 4 ) 2 , is found in all tissues, in associa- 

 tion with calcium phosphate, though in much smaller quantity. 



Magnesium carbonate, MgCO 3 , occurs only in traces in the blood. 



IRON COMPOUNDS. 



Iron is a constituent of the coloring-matter of the blood. Traces, how- 

 ever, are also found in lymph, bile, gastric juice, and in the pigment of the 

 eyes, skin and hair. The amount of iron contained in a body weighing 70 

 kilograms is about 2.2 gm. It exists under various forms e.g., ferric oxid, 

 and in combination with organic compounds. 



lodin is found in a number of organs thyroid gland, lungs, ovaries, 

 liver, hypophysis, small intestines as well as in the blood and bile. The 

 iodine content of the thyroid gland is eight to ten times that of all the other 

 organs. The active ingredient of the thyroid colloid is the iodin-containing 

 substance called thyroidin. It is contained in that portion of the protein 

 which is soluble in physiological salt solution and precipitated upon half 

 saturation with ammonium sulphate. This portion is apparently a globulin 

 called thyreoglobulin, and contains an easily separated carbohydrate group. 

 The thyroid glands of new-born children are iodin free. 



Chemic analysis thus shows that the chemic elements into which the 

 compounds may be resolved by an ultimate analysis do not exist in the 

 body in a free state, but only in combination, and in characteristic pro- 

 portions, to form compounds whose properties are the resultant of those of 

 the elements. Of the four principal elements which make up 97 per cent, 

 of the body, O, H, N are extremely mobile, elastic, and possessed of great 

 atomic heat. C, H, N are distinguished for the narrow range of their 

 affinities, and for their chemic inertia. C possesses the greatest atomic 

 cohesion. O is noted for the number and intensity of its combinations. 



As the properties of the compounds formed by the union of elements 

 must be the resultants of the properties of the elements themselves, it follows 

 that the ternary compounds, starches, sugars, and fats must possess more 

 or less inertia, and at the same time instability; while in the more complex 

 proteids, in which sulphur and phosphorus are frequently combined with 

 the four principal elements, molecular instability attains its maximum. 

 As all the foregoing compounds possess in varying degrees the properties of 

 inertia and instability, it follows that living matter must possess correspond- 

 ing properties, and the capability of undergoing unceasingly a series of 

 chemic changes, both of composition and decomposition, in response to the 

 chemic and physical influences by which it is surrounded, and which underlie 

 all the phenomena of life. 



