MAINTENANCE BY NUTRITION. 



379 



mechanical external force the natural termination of a 

 certain period of life. -Yet, before the hair dies, provision 

 is made for its successor : for when its growth is failing, 

 there appears below its base a dark spot, the germ or 

 young pulp of the new hair 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 pro- 

 cess of successive life and death, and life communicated to 

 a successor, 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 internal 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 develop- 

 ed 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 

 perfection, retains for a time 

 its perfect state, and still 

 lives, though it does not grow. 

 But at length, as the new tooth 

 comes, the deciduous tooth 

 dies ; or rather its crown dies, 

 and is cast out like the dead hair, while its fang, with 

 its bony sheathing, and vascular and nervous pulp, de- 

 generates and is absorbed (fig. 103). The degeneration is 



* Fig. 103. 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. 



