MAINTENANCE EY NUTRITION. 381 



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, shedding of antlers, of desquamation, change of 

 plumage in birds, and of hair in Mammalia, the only ex- 

 planation 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 developed to perfection, to 

 live their life in the mature state, and in their turn to be 

 cast off. So also, in some elementary structures we may 

 discern the same laws of determinate period of life, death, 

 or degeneration, and replacement. They are evident in the 

 history of the blood-corpuscles, both in the superseding 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-cor- 

 puscles (see p. 92). 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 m each particle is, however, liable to 

 be modified ; especially by the exercise of the function of 

 the part. The less a part is exercised the longer do its 

 component particles appear to live : the more active its 

 functions are, the less prolonged is the existence of its 

 individual particles. So in the case of single cells ; if the 

 general development of the tadpole be retarded by keeping 

 it in a cold, dark place, and if hereby the function of the 

 blood-corpuscles be slowly and imperfectly discharged, 

 they will maintain their embryonic state for even several 

 weeks later than usual, the development of the second set 



