ANIMAL ORGANIZATION. 



117 



are no longer susceptible of being nourished, 

 and from the moment of their deposition, un- 

 dergo no further change, except from the action 

 of external agents. By the continual additions 

 that are made to them at their base, or root, 

 where the vessels deposit fresh materials, they 

 gradually increase in size, protrude through the 

 skin, and continue to grow by the same process, 

 as long as these vessels continue in activity. 



The nature of this process is well exemplified 

 in the growth of hair. Fig. 32 shows the appa- 

 ratus employed in its construction, in an imagi- 

 nary section of the root, on a magnified scale. 

 Every hair takes its rise from a minute vascular 

 pulp, P, of an oval shape, which is implanted 

 below the corium, or true skin, D.* This pulp is 



invested by a sheath 

 or capsule, c, which, 

 together with the con- 

 tained pulp, and the 

 root of the hair that 

 grows from it, com- 

 poses the bulb of the 

 hair. The bulb itself 

 is contained in a small 

 cell formed by con- 

 densed membranes, s, 

 to which it has no 



* In the above figure E is a section of the Epidermis, or cuticle ; 

 the dotted part, R, represents the situation of the subjacent rete 

 mitcoaum, and D, the derm, or corium. 



