1 10 THE MECHANICAL FUNCTIONS, 



the apparatus by which the nutrient particles 

 are selected, combined and elaborated into the 

 materials of the intended structure. The original 

 form, situation, and disposition of this vascular 

 pulp, determines the future figure and extent of 

 growth of the production which is to arise from 

 it. The materials which compose it are depo- 

 sited sometimes in masses, as in the scales of 

 the crocodile ; more generally in layers, as in 



30 



31 



hoofs and nails, and also in the scales of 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, 



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

 tinguished by applying to them a high magnifying power. As 

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

 project further, 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 fluriatilis. 

 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. 



