104 THE MECHANICAL FUNCTIONS. 



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

 and the scales of fishes, reptiles, and other ani- 

 mals.* The integuments of insects, and especially 

 their more solid and horny coverings, contain, how- 

 ever, as will hereafter be noticed, a peculiar che- 

 mical principle termed Entomoline. 



All these parts seem to be but remotely connected 

 with the vital actions of the system with which they 

 are associated ; and it is doubtful how far they are 

 to be considered as appertaining to the living por- 

 tion of the body, or as mere extraneous appendages. 

 Yet, however they may differ in their forms, uses, 

 and external appearance, they all are produced by 

 the same kind of vascular or glandular structure, 

 variously arranged to suit the particular circum- 

 stances in each case : and the mode of their 

 developement and growth is essentially the same 

 in all. 



An extremely delicate and finely organized pulp, 

 composed 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 dispo- 

 sition of this vascular pulp determine the future 

 figure and extent of growth of the production which 

 is to arise from it. The materials which 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 



* The scales of the greater number of fossil fishes contain a large 

 quantity of phosphate of lime ; and may therefore be regarded as 

 having the character of bone. 



