NUTRITION. 299 



CHAPTER XI. 



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 com- 

 ponent particles of the tissues is repaired ; and each elementary 

 particle seems to have the power not only of attracting ma- 

 terials from the blood, but of causing them to assume its struc- 

 ture, and participate in its vital properties. 



The relations between development and growth have been 

 already stated (Chap. I) ; 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, at- 

 tained ; and this, notwithstanding continual changes in their 

 component particles. It is by this process that an adult per- 

 son, in health, is maintained, 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 maintained. Every 

 organ or part of the body, as much as the whole, exactly main- 

 tains its form and composition, as the issue of the changes con- 

 tinually taking place among its particles. 



The change of component particles, in which the nutrition 

 of organs consists, is most evidently shown when, in growth, 

 they maintain their form and other general characters, but 

 increase in size. When, for example, a long bone increases 

 in circumference, and in the thickness of its walls, while, at 

 the same time, its medullary cavity enlarges, it can only be 

 by the addition of materials to its exterior, and a coincident 

 removal of them from the interior of its wall ; and so it must 

 be with the growth of even the minutest portions of a tissue. 

 And that a similar change of particles takes place, even while 



