38 PHYSIOLOGY CHAP. 



it dissolves the single particles, and renders them capable of being 

 transported. It is present partly in chemical combination, partly 

 as solvent for the various substances of the cell-contents. The 

 amount by weight of water in the tissues is on an average over 

 50 per cent. According to von Bezold, the total content of water 

 in the human body is about 59 per cent. Bone contains 22 per 

 cent water, liver 69 per cent, muscle 75 per cent, the kidneys 82 

 per cent. 



The water holds in solution a number of salts, which are never 

 wanting in living substance. Chlorides largely predominate ; next 

 come the carbonates, sulphates, phosphates of the alkalies and 

 alkaline earths. Such are the chlorides of sodium, potassium, and 

 ammonium ; the carbonates, sulphates, and phosphates of sodium, 

 potassium, calcium, magnesium, and ammonium. A considerable 

 part of these salts is probably in chemical combination with the 

 organic substances. 



The gases, oxygen, carbonic acid, and nitrogen, when not 

 chemically combined, are simply dissolved in the water ; very 

 occasionally they occur in the form of gaseous vesicles, as in 

 certain unicellular Ehizopods. 



XII. After this bird's-eye review of the vast province of the 

 chemistry of elementary organisms, undertaken solely with the 

 object of classifying into groups and subgroups the several bodies 

 that compose the substratum of the phenomena of life, it must 

 again be emphasised that we are far from any adequate knowledge 

 of the chemical structure of living matter. It is impossible to 

 investigate this living matter without first killing it, i.e. destroying 

 its vitality. The chemical compounds, organic and inorganic, 

 which we have seen to exist in plants and animals, are only the 

 products of this destruction, i.e. they represent the chemical 

 aggregates, which can be recognised and isolated from the dead 

 body. They certainly exist in the cell ; but we are entirely 

 ignorant of the mode in which they are associated and combined 

 among themselves, so as to compose the living matter. Nor 

 should this surprise us, when we reflect that with the ordinary 

 methods of chemical analysis we have no means of ascertaining 

 the exact chemical nature of the individual salts contained, e.g. in 

 a mineral water. We can only determine the quality and quantity 

 of the acids and bases contained in it ; as to what these salts are, 

 and how they are mixed together, we know nothing. Any state- 

 ments in regard to this are mere guesswork. 



The physiologist needs to be very circumspect and cautious in 

 applying the data thus derived from the chemistry of dead matter 

 to the phenomena of living substance, in which the chemical 

 relations of the several molecular aggregates are very different, and 

 the molecules themselves are highly complex and excessively 

 unstable. 



