379 



CHAPTER XT. 



NUTRITION AND GROWTH. 



NUTRITION or nutritive assimilation is that modification of 

 the formative process peculiar to living bodies by which 

 tissues and organs already formed maintain their integrity. 

 By the incorporation of fresh nutritive principles into their 

 substance, the loss consequent on the waste and natural 

 decay of the component particles of the tissues is repaired ; 

 and each elementary particle seems to have the power not 

 only of attracting materials from the blood, but of causing 

 them to assume its structure, and participate in its vital 

 properties. 



The relations between development, growth, and simple 

 nutrition or maintenance, have been already stated (Chap. 

 I. and II.) ; under the head of NUTRITION will be now 

 considered the process by which parts are maintained in 

 the same general conditions of form, size, and composition, 

 which they have already, by development and growth, 

 attained ; and this, notwithstanding, but rather by means 

 of, continual changes in their component particles. It is 

 by this process that an adult person, in health, is main- 

 tained, through a series of some years, with the same 

 general outline of features, the same size and form, and 

 perhaps even the same weight ; although, during all this 

 time, the several portions of his body are continually 

 changing : their particles decaying and being removed, 

 and then replaced by the formation of new ones, which, 

 in their turn, also die and pass away. Neither is it only 

 a general similarity of the whole body which is thus main- 

 tained. Every organ or part of the body, as much as the 

 whole, exactly maintains its form and composition, as 



