24: INTEODUCTIOX. 



which they enter. Others are consumed in the process of 

 repair, and after having performed their functions, are thrown 

 off as effete matters. Examples of the latter are fibrin 

 and albumen, which are transformed first into the sub- 

 stance of the tissues, and then into urea, creatine, choleste- 

 rine, and other excrementitious matters, which are the re- 

 sult of the breaking down or wearing out of the tissues. 

 Finally, there are certain principles, the sugars and fats for 

 example, which have an important connection with the pro- 

 cess of nutrition, and disappear in the system, but whose 

 transformations we have not as yet been able to follow. 

 These, besides being taken in as food, are manufactured by 

 certain organs, and appear de novo in the economy. 



Division of Proximate Principles. In the division of 

 proximate principles, we shall follow, with slight modifica- 

 tions, the classification of Robin and Yerdeil. With refer- 

 ence solely to anatomical and physiological chemistry, the 

 classification of these authors cannot be improved; but in 

 treating of the whole subject of physiology, it will be conven- 

 ient to take up certain of the elements in connection with the 

 functions in which they play an important part. Oxygen 

 and carbonic acid, for example, will be fully considered in 

 connection with respiration urea and cholesterine with ex- 

 cretion, &c. Again, there are some whose function is appa- 

 rently of so little importance, or so obscure, that, while they 

 may be interesting in a chemical point of view, merely as 

 constituents of the body, it is not worth while to treat of 

 them in connection with physiology. 



The two great divisions of proximate principles which we 

 propose, comprise : 



FIEST. /Substances which enter into the normal con- 

 stitution of the organized tissues, and those constituents of 

 the fluids which are used in nutrition. 



