ANIMAL ORGANIZATION. 



105 



fishes;* and occasionally in filaments, as in hair; 

 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 example. In all cases, the portions 

 thus successively produced, are no longer suscep- 

 tible of being nourished, and from the time of their 

 deposition, undergo no further change, except from 

 the action of external agents. t By the continual 

 additions which 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 apparatus 



* The laminated structure of the scales of fishes is easily distin- 

 guished by applying to them a high magnifying power. As the 

 breadth of each new layer is greater than the last, its edges project 

 farther, the whole surface having that concentric striated appearance 

 which renders it an interesting object for microscopic examination. 

 Fig. 29 exhibits the striated surface of the scale of the Cyprinus 

 alburnus, and Fig. 30 that of the Perca Jiuviatilis. The imbricated 

 arrangements 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. 



f Dr. Mandl takes a very different view of the growth of the 

 scales of fishes, considering it as effected by an internal nutrition. 

 Comptes Rendus,x. 338, and Edin. Phil. Journal, xxviij. 126 and 274. 



