94 INTRODUCTION. 



tion that they have shown them all to contain nitrogen, are 

 almost without value. Without exception they are all in a 

 state of intimate molecular union with inorganic matter, and 

 in this union inorganic compounds become endowed with 

 life ; that is, the inorganic parts of the body, as the calcareous 

 elements of bone, taken up by the blood with the worn-out 

 organic principles and undergoing constant waste, are capa- 

 ble of self-regeneration. 



The vitality thus imparted to inorganic matters, and the 

 fact that neither the organic nor inorganic elements are alone 

 capable of engaging in the phenomena of life, cannot ~be too 

 fully insisted upon. Both are taken into the body as food, 

 are digested, assimilated, and finally discharged, always in 

 combination; the organic principles changed, and converted 

 into excrementitious substances, and the inorganic principles 

 unchanged. 



The readiness with which the organic principles are con- 

 verted one into the other by catalysis must also be appre- 

 ciated, as well as the constant operation of this process in 

 all the phenomena of life. Even albumen, taken in as food, 

 must be converted into albuminose, and again into albumen, 

 before it is capable of building up the tissues ; and all the 

 nitrogenized articles of food are converted into the same .sub- 

 stance, regenerating the blood, and through it the body. 



In the economy we find two great divisions of organic 

 elements: one, which is nutritive, and the other, which 

 forms the great part of the tissues. By simple contact, the 

 plastic, or nutritive, principles are mysteriously converted 

 into the varied elements of the organism, and take with them 

 the inorganic elements necessary to the proper constitution 

 of the parts. 



It is only with a just appreciation of these general princi- 

 ples that we are able to study intelligently the special functions 

 of respiration, circulation, digestion, absorption, secretion and 

 excretion, which are all tributary to the complicated function 

 of nutrition. 



