92 



THE MECPIANICAL FUNCTIONS, 



nails, the hoofs, and the claws of quadrupeds, and the scales 

 of fishes, reptiles, and other animals. The integuments of 

 insects, and especially their more solid and horny coverings, 

 contain, however, as will hereafter be noticed, a peculiar 

 chemical principle termed Entornoline. 



All these parts seem to be but remotely connected with 

 the vital actions of the system with which they are associ- 

 ated; and it is doubtful how far they are to be considered as 

 appertaining to the living portion of the body, or as mere 

 extraneous appendages. Yet, however they may difier in 

 their forms, uses, and external appearance, they all are pro- 

 duced by the same kind of vascular structure, variously ar- 

 ranged to suit the particular circumstances in each case: and 

 the mode of their development and growth is essentially the 

 same in all. 



An extremely delicate and finely organized pulp, com- 

 posed partly of a congeries of minute vessels, and partly of 

 a gelatinous substance, in which these vessels are embedded, 

 constitutes the apparatus by which the nutrient particles are 

 selected, combined and elaborated into the materials of the 

 intended structure. The original form, situation, and dis- 

 position of this vascular pulp, determines the future figure 

 and extent of growth of the production which is to arise 

 from it. The materials w^iich compose it are deposited 

 sometimes in masses, as in the scales of the crocodile; more 



generally in layers, as in hoofs and nails, and also in the 

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



• The laminated structure of the scales of fishes is easily distinguished 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 



