ANIMAL ORGANIZATION. 



93 



which latter, again, are often agglutinated together by a 

 strong cement, uniting them into a hard and solid structure, 

 of which the horn of the rhinoceros is a remarkable exam- 

 ple. In all cases, the portions thus successively produced, 

 are no longer susceptible of being nourished, and from the 

 moment of their deposition, undergo no farther change, ex- 

 cept from the action of external agents. By the continual 

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

 the vessels deposite 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 acti- 

 vity. 



The nature of this process is well exemplified in the 

 growth of hair. Fig. 32 shows the apparatus employed in 

 its construction, in an imaginary 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, p.* This pulp is invested by a 



sheath or capsule, c, which, 

 together with the contained 

 pulp, and the root of the hair 

 that grows from it, composes 

 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 attachment 

 excepting at the lower part, v, 

 where the vessels and nerves 

 of the pulp are passing into 



having that concentric striated appearance which renders it an interesting* ob- 

 ject for microscopic cxaniination. Fig. 29 exhibits the striated surface of the 

 scale of the Cyprinxis alburnus, and Fig. 30 that of the Perca JluviatiUs. 

 The imbricated arrangement of these scales, resembling- that of the tiles on 

 the roof of a house, is shown in Fig. 31. All these figures represent the 

 objects highly magnified. 



* In the above figure r. is a section of the Epidermis, or cuticle; the dotted 

 part, R, represents tiic situation of the subjacent retc mucosum, and d, the 

 derm, or corium. 



