ANIMAL ORGANIZATION. 107 



(as shown at b), without ever succeeding in reach- 

 ing 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 outward integument, which, 

 stretching as the hair increases in length, forms 

 over it a very fine external tunic. But later obser- 

 vations 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 ac- 

 companying 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 origin, 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 surface 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 sup- 

 plying nutriment become exhausted. The first 

 deficiency in its nourishment appears in the cessa- 

 tion of the deposit of colouring matter, and the 

 hair in consequence becomes grey. After a time, 

 the vessels becoming quite impervious, the bulb 

 shrivels, the hair is detached, and the canal which 

 its root occupied in the skin becomes obliterated. 



The hair of different animals, and sometimes 

 even of different parts of the same animal, varies 

 in shape, texture, and mechanical properties. 



