306 NUTRITION. 



hind is not formed of germs derived from the row before : the 

 front row is simply repeated in the second one, the second in 

 the third, and so on. So, in cuticle, the deepest layer of epi- 

 dermis-cells derives no germs from the layer above : their de- 

 velopment is not like a reproduction of the cells that have gone 

 on towards the surface before them : it is only a repetition. It 

 is not improbable that much of the difference in the degree of 

 repair, of which the several tissues are capable after injuries 

 or diseases, may be connected with these differences in their 

 ordinary mode of nutrition. 



In order that the process of nutrition may be perfectly ac- 

 complished, certain conditions are necessary. Of these, the 

 most important are : 1. A right state and composition of the 

 blood, from which the materials for nutrition are derived. 2. 

 A regular and not far distant supply of such blood. 3. A cer- 

 tain influence of the nervous system. 4. A natural state of 

 the part to be nourished. 



1. This right condition of the blood does not necessarily im- 

 ply its accordance with any known standard of composition, 

 common to all kinds of healthy blood, but rather the existence 

 of a certain adaptation between the blood and the tissues, and 

 even the several portions of each tissue. Such an adaptation, 

 peculiar to each individual, is determined in its first formation, 

 and is maintained in the concurrent development and increase 

 of both blood and tissues ; and upon its maintenance in adult 

 life appears to depend the continuance of a healthy process of 

 nutrition, or, at least, the preservation of that exact sameness 

 of the whole body and its parts, which constitutes the perfec- 

 tion of nutrition. Some notice of the maintenance of this 

 sameness in the blood has been given already (p. 84), in 

 speaking of the power of assimilation which the blood exer- 

 cises, a power exactly comparable with this of maintenance by 

 nutrition in the tissues. And evidence of the adaptation be- 

 tween the blood and the tissues, and of the exceeding fineness 

 of the adjustment by which it is maintained, is afforded by the 

 phenomena of diseases, in which, after the introduction of cer- 

 tain animal poisons, even in very minute quantities, the whole 

 mass of the blood is altered in composition, and the solid tis- 

 sues are perverted in their nutrition. It is necessary to refer 

 only to such diseases as syphilis, small -pox, and other erup- 

 tive fevers, in illustration. And when the absolute dependence 

 of all the tissues on the blood for their very existence is re- 

 membered, on the one hand, and, on the other, the rapidity 

 with which substances introduced into the blood are diffused 

 into all, even non-vascular textures (p. 297), it need be no 

 source of wonder that any, even the slightest alteration, from 



