PROXIMATE PRINCIPLES. 23 



properties, and is endowed with certain functions in the econ- 

 omy, which are, of course, entirely different from the proper- 

 ties of chlorine or sodium ; the latter especially being only 

 obtained in a state of chemical purity by a difficult and elab- 

 orate process of manipulation. As physiologists we have 

 nothing to do with the properties of chlorine, or the rare 

 metal sodium ; we only wish to know as much as possible 

 about the functions of these two bodies united to form com- 

 mon salt. Again, fibrin, a proximate principle found in the 

 blood, may be reduced by chemical manipulations to a cer- 

 tain number of atoms of carbon, hydrogen, oxygen, nitrogen, 

 and sulphur. But a knowledge of even the exact proportions 

 of these ingredients would be of no practical benefit, if we 

 were unacquainted with the general properties of fibrin and 

 its uses in the economy. Salt cannot be subdivided into 

 chlorine and sodium, nor fibrin into its elements, without 

 chemical decomposition and loss of characteristic proper- 

 ties but both of these substances can be extracted from the 

 body in the condition in which they exist in the organism, 

 and are therefore proximate principles. 



A constituent of the body may be at the same time a 

 chemical element and a proximate principle. An example 

 of this is the free oxygen in solution in the blood. This 

 enjoys, in the body, the properties of free oxygen, and may 

 be extracted from the blood by mere displacement with an- 

 other gas, or by the air-pump ; a process quite different from 

 the elaborate chemical manipulation which would be neces- 

 sary to obtain oxygen by decomposition of fibrin, albumen, 

 or any compound principle. 



The principles which compose the body, with the excep- 

 tion of excrementitious substances, exist in our food ; this 

 being the only way in which material is supplied for the con- 

 tinual repair which is characteristic of living tissues. They 

 are all introduced from without. Certain principles, such as 

 water and the inorganic salts, are merely transitory in the in- 

 terior of the body, and are discharged in the same form in 



