CHEMICAL INGREDIENTS OF THE BODY. 38 



other hand, potassium chloride is more abundant in the muscles, less 

 so in the blood. But these proportions are nowhere absolute or in- 

 variable. There is a difference, in this respect, between the chemical 

 composition of an inorganic substance and the physiological constitu- 

 tion of an animal fluid. The former is constant and definite ; the latter 

 always presents certain variations. Thus, water is invariably com- 

 posed of the same relative quantities of hydrogen and oxygen ; and 

 these proportions are essential to its existence. But in the urine, the 

 proportions of water, urea, urates, and phosphates vary within certain 

 limits in different individuals, and even in the same individual, from 

 one hour to another. This physiological variation takes place, within 

 the limits of health, in all the animal solids and fluids. It is a necessary 

 accompaniment of the actions of life, and one of the characteristic phe- 

 nomena of living beings. The animal body is the seat of incessant 

 changes, and all its manifestations of vital activity are either the 

 causes or the result of its internal alterations. Every variation in its 

 general condition is accompanied by a corresponding variation in the 

 constitution of its different parts. This constitution is consequently of 

 a very different character from the chemical constitution of an oxide or a 

 salt. In the analysis of an animal tissue or fluid, the numbers express- 

 ing the proportion of its different ingredients are always understood to 

 be approximate, and not absolute. They represent the general character 

 of the mixture, but allow of its variation within physiological limits. 



The chemical ingredients of the body are naturally divided into five 

 classes : 



The first of these classes comprises all ingredients of a purely 

 INORGANIC nature. These substances are derived mostly from the exte- 

 rior. They are found abundantly in the inorganic world as well as 

 in organized bodies ; and they present themselves under the same form 

 and with the same properties in the interior of the animal frame as 

 elsewhere. They are crystallizable, with definite chemical characters 

 and a simple chemical constitution. They are compounds, in simple 

 proportions, of hydrogen and oxygen, the metals of the alkaline and 

 earthy salts, sulphur, phosphorus, chlorine, and, in general terms, of 

 the ingredients of mineral substances. They comprise water, which is 

 the most abundant of its class in the animal frame, sodium and potas- 

 sium chlorides, phosphates, and sulphates, alkaline carbonates, the salts 

 of lime and magnesia, together with combinations of a few other metallic 

 elements in small quantity. 



The second class consists of the HYDRO-CARBONACEOUS SUBSTANCES of 

 organic origin. They are distinguished from inorganic matters first by 

 the fact of their containing carbon in large proportion as one of their 

 immediate constituents, associated with hydrogen and oxygen, but 

 with no other chemical element. They are either crystallizable or 

 readily convertible into other crystallizable members of the same group. 

 Their chemical composition is less simple than that of inorganic sub- 

 stances, but it is still sufficiently definite, and their chemical characters 



C 



