MAINTENANCE BY NUTRITION. 



303 



FIG. 103. 



covered with cells containing pigment, and often connected by 

 a series of pigment-cells with the old pulp or capsule (Fig. 

 102, B). 



Probably there is an intimate analogy between the process 

 of successive life and death, and life communicated to a suc- 

 cessor, which is here shown, and that which constitutes the 

 ordinary nutrition of a part. It may be objected, that the 

 death and casting out of the hair cannot be imitated in inter- 

 nal parts ; therefore, for an example in which the assumed 

 absorption of the worn-out or degenerate internal particles is 

 imitated in larger organs at the end of their appointed period 

 of life, the instance of the deciduous or milk-teeth may be 

 adduced. 



Each milk-tooth is developed from its germ ; and in the 

 course of its own development, separates a portion of itself to 

 be the germ of its successor; and 

 each, having reached its perfec- 

 tion, retains for a time its perfect 

 state, and still lives, though it 

 does not grow. But at length, 

 as the new tooth comes, the de- 

 ciduous tooth dies ; or rather its 

 crown dies, and is cast out like 

 the dead hair, while its fang, 

 with its bony sheathing, and vas- 

 cular and nervous pulp, degen- 

 erates and is absorbed (Fig. 103). 

 The degeneration is accompanied 

 by some unknown spontaneous 

 decomposition of the fang ; for it 

 could not be absorbed unless it 

 was first so changed as to be solu- 

 ble. And it is degeneration, not 

 death, which precedes its re- 

 moval; for when a tooth-fang 



dies, as that of the second tooth does in old age, then it is not 

 absorbed, but cast out entire, as a dead part. 



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

 cess 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 composition, 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 cuti- 



Section of a portion of the upper 

 jaw of a child, showing a new tooth 

 in process of formation, the fang of 

 the corresponding deciduous tooth 

 being absorbed. 



