MAINTENANCE BY NUTRITION. 385 



accompanied by some unknown spontaneous decomposition 

 of the fang ; for it could not be absorbed unless it was 

 first so changed as to be soluble. And it is degeneration, 

 not death, which precedes its removal ; for when a tooth- 

 fang dies, as that of the second tooth does in old age, then 

 it is not absorbed, but is cast out entire, as a dead part. 



Such, or generally such, it seems almost certain, is the 

 process of maintenance by nutrition ; the hair and teeth 

 may be fairly taken as types of what occurs in other parts, 

 for they are parts of complex organic structure and com- 

 position, and the teeth-pulps, which are absorbed as well 

 as the fangs, are very vascular and sensitive. 



Nor are they the only instances that might be adduced. 

 The like development, persistence for a time in the perfect 

 state, death, and discharge, appear in all the varieties of 

 cuticles and gland-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 exercise 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, 



cc 



