94 THE MECHANICAL FUNCTIONS. 



it. The hair, growing by depositions from the inside of the 

 capsule, which forms the outer part, o, of the shaft, and from 

 the outside of the pulp, w^hich forms the inner or central 

 part, I, is forced upwards till it has pierced the skin; and in 

 the course of its passage a canal is formed for it in the skin 

 itself, and continuous with that which encloses the bulb: 

 and the course of this canal is generally oblique. In the ele- 

 phant, where the thickness and density of the hide, present 

 considerable obstacles to the passage of the hairs through it, 

 we may discover, on minute examination, many hairs that 

 have only penetrated a certain way, as shown at b, without 

 ever succeeding in reaching the surface. 



An opinion has been very commonly entertained that 

 each hair, on its protruding from underneath the cuticle, 

 E, at the point q, carries up along with it a portion of this 

 outer integument, which, stretching as the hair increases in 

 length, forms over it a very fine external tunic. But later 

 observations have shown that this is not the case, and that 

 there is simply an adhesion of the edge of the cuticle to the 

 origin of the hair, without any accompanying prolongation; 

 so that if the whole bulb be destroyed, and its pulp absorbed, 

 the hair may be detached by the slightest force. 



From this account it will be seen that a hair is, in its ori- 

 gin, tubular; the inner part being occupied by the pulp. 

 But as the pulp extends only to that portion of the hair 

 which is in a state of growth, it never rises above the sur- 

 face of the skin; and the cavity in the axis of the hair is 

 either gradually obliterated, or is filled with a dry pith, or 

 light spongy substance, probably containing air. After a 

 certain period, the bulb diminishes in size, from the collapse 

 of the vessels whose powers of supplying nutriment become 

 exhausted. The first deficiency in its nourishment appears 

 in the cessation of the deposite of colouring matter, and the 

 hair in consequence becomes gray. After a time, the ves- 

 sels becoming quite impervious, the bulb shrivels, the hair 

 is detached, and the canal which its root occupied in the 

 skin becomes obliterated. 



