SUBSTANCES WHICH PASS THROUGH THE ORGANISM. 429 



rides, have important uses of a purely physical character. It is necessary, for 

 example, that the blood should contain a certain proportion of sodium chlo- 

 ride, this substance modifying and regulating the processes of absorption and 

 probably of assimilation. In addition, however, the chlorides exist as con- 

 stituent parts of every tissue and organ of the body, and they are so closely 

 united with the nitrogenized matters that they can hot be completely sepa- 

 rated without incineration. Those inorganic matters, the uses of which are 

 so important in their passage through the body, are found largely as con- 

 stituents of the fluids and are less abundant in the solids. They are con- 

 tained in large proportion, also, in the liquid excretions ; and any excess over 

 the quantity actually required by the system is thrown off in this way. Other 

 inorganic matters are specially important as constituent parts of the tissues, 

 and they are more abundant in the solids than in the fluids. Examples of 

 substances of this class are the calcium salts, particularly the phosphates. 

 These are also in a condition of intimate union with organic matters. 



If certain simple chemical changes be excepted, such as the decomposi- 

 tion of the bicarbonates, the inorganic constituents of food do not necessarily 

 undergo any modification in digestion. They are generally introduced already 

 in combination with organic matters, and they accompany them in the 

 changes which they pass through in digestion, assimilation by the blood, 

 deposition in the tissues, and the final transformations that result in the 

 various excrementitious products ; so that the inorganic salts are found united 

 with the organic matter of the food as it enters the body, and what seem to 

 be the same substances, in connection with the organic excrementitious mat- 

 ters. Between these two conditions, however, are the various operations of 

 assimilation and disassimilation, or metabolism, from which inorganic matters 

 are never absent. 



Inorganic Constituents of the Body. The number of inorganic substances 

 now well established as existing in the human body is about twenty-one ; but 

 some are found in small quantities, are not always present and apparently 

 have no very important uses. These will be passed over rapidly, as well as 

 those which are so intimately connected with some important function as to 

 render their full consideration in connection with that function indispensable. 



Gases. The gases (oxygen, hydrogen, nitrogen, carburetted hydrogen 

 and hydrogen monosulphide) exist both in a gaseous state and in solution in 

 some of the fluids of the body. Oxygen plays a most important part in the 

 function of respiration ; but the office of the other gases is by no means so 

 essential. Nitrogen seems to be formed by the system in small quantity and 

 is taken up by the blood and exhaled by the lungs, except during inanition, 

 when the blood absorbs a little from the inspired air. It exists in greatest 

 quantity in the intestinal canal. Carburetted hydrogen and hydrogen mono- 

 sulphide, with pure hydrogen, are found in minute quantities in the expired 

 air and exist in a gaseous state in the alimentary canal. From the offensive 

 nature of the contents of the large intestine, one would suspect the presence 

 of hydrogen monosulphide in considerable quantity ; but actual analysis has 

 shown that the gas contained in the stomach and in the small and large in- 

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