INORGANIC BODIES. 73 



soluble, or otherwise facilitating tissue change. The salts pass 

 out of the body in many secretions, notably in the urine, where 

 they have great influence on the elimination of urea, and therefore 

 form a most important constituent of that secretion. 



Common Salt (Sodium Chloride}, NaCl, is the most widely 

 spread, and is present in greater quantity than any other salt in 

 all fluids and tissues, except in bones, teeth, red blood corpuscles, 

 and red muscle. 



Potassium Chloride commonly accompanies sodium chloride in 

 small quantity. In the red blood corpuscles and in muscle it 

 occurs in greater amount than the sodium salt, while in the blood 

 plasma but little is found in comparison with the soda salts, and 

 any excess seems to act as a poison to the heart. 



Carbonates and phosphates of calcium, sodium, potassium and 

 magnesium occur in small quantities in most tissues. The earthy 

 part of bone is chiefly composed of calcium and magnesium phos- 

 phate and calcium carbonate, together with some calcium fluoride. 



Sulphates of sodium and potassium, probably formed in the body 

 from the oxidization of the sulphur in the complex proteid mate- 

 rials, occur in most tissues, and are removed from the body by 

 the kidneys. 



Finally we find two of the elements free in the textures. Of 

 these Oxygen plays by far the most important part. It is widely 

 distributed among the fluids of the body, from which it can be 

 removed by reducing the pressure of the oxygen of the atmosphere 

 by means of an air pump. Oxygen is introduced into the body 

 by the lungs, where the blood takes it from the air. In the blood 

 only a small quantity of that which can be removed by the air- 

 pump is really free, the remainder is chemically combined with 

 the coloring matter of the blood. It is absolutely necessary for 

 life, as it alone can enable the chemical changes of the tissues, 

 which are mostly oxidizations, to go on. It is, in fact, the element 

 necessary for the slow combustion which takes place in the nutri- 

 ent material after its assimilation. 



Nitrogen also occurs in the blood, but in insignificant quantity. 

 It is absorbed from the atmosphere as the blood passes through 

 the lungs. So far as we know, it has no physiological importance 

 in the body. 



