304 NUTRITION. 



cles and glaDd-cells ; and in the epidermis, as in the teeth, 

 there is evidence of decomposition of the old cells, in the fact 

 of the different influence which acetic acid and potash exer- 

 cise on them and on the young cells. Seeing, then, that the 

 process of nutrition, as thus displayed, both in active organs 

 and in elementary cells, appears in these respects similar, the 

 general conclusion may be that, in nutrition, the ordinary 

 course of each complete elementary organ in the body, after 

 the attainment of its perfect state by development and growth, 

 is to remain in that state for a time ; then, independently of 

 the death or decay of the whole body, and in some measure, 

 independently of its own exercise, or exposure to external 

 violence, to die or to degenerate ; and then, being cast out or 

 absorbed, to make way for its successor. 



It appears, moreover, that the length of life which each part 

 is to enjoy is fixed and determinate, though in some degree 

 subject to accidents and to the expenditure of life in exercise. 

 It is not likely that all parts are made to last a certain and 

 equal time, and then all need to be changed. The bones, for 

 instance, when once completely formed, must last longer than 

 the muscles and other softer tissues. But when we see that the 

 life of certain parts is of determined length, whether they be 

 used or not, we may assume, from analogy, the same of 

 nearly all. 



Now, the deciduous human teeth have an appointed average 

 duration of life. So have the deciduous teeth of all other 

 animals; and in all the numerous instances of moulting, shed- 

 ding of antlers, of desquamation, change of plumage in birds, 

 and of hair in Mammalia, the only explanation is that these 

 organs have their severally appointed times of living, at the 

 ends of which they degenerate, die, are cast away, and in due 

 time are replaced by others, which, in their turn, are to be de- 

 veloped to perfection, to live their life in the mature state, and 

 in their turn to be cast off. So also, in some elementary struc- 

 tures, we may discern the same laws of determinate period of 

 life, death, or degeneration, and replacement. They are evi- 

 dent in the history of the blood-corpuscles, both in the super- 

 seding of the first set of them by the second at a definite period 

 in the life of the embryo, and in the replacement of those that 

 degenerate by others new-formed from lymph -corpuscles. (See 

 p. 83.) And if we could suppose the blood-corpuscles grouped 

 together in a tissue instead of floating, we might have in the 

 changes they present an image of the nutrition of the elements 

 of the tissues. 



The duration of life in each particle is, however, liable to be 

 modified ; especially by the exercise of the function of the 



