388 NUTRITION. 



which we speak of the organs by which new individuals 

 are formed, as the reproductive organs. But in the shark's 

 jaws, and others, in which we see row after row of teeth 

 succeeding each other, the row behind 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 epidermis- 

 cells derives no germs from the layer above them ; their 

 development 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 con- 

 nected with these differences in their ordinary mode of 

 nutrition. 



In order that the process of nutrition may be perfectly 

 accomplished, certain conditions are necessary. Of these, 

 the most important are : i . 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 certain influence of the nervous system. 4. A 

 natural state of the part to be nourished. 



i. This right condition of the blood does not necessarily 

 imply its accordance with any known standard of com- 

 position, 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 perfection 

 of nutrition. Some notice of the maintenance of this 

 sameness in the blood has been given already (p. 104), in 



