INTRODUCTION. ^ 



most imperfect genera the nervous substance seems 

 mixed up and confounded with the general substance 

 of the body. 



When a volition, or determination of the will is 

 excited in an animal, by means of sensation or 

 otherwise, it is, as has been observed, communi- 

 cated to the muscles, through the medium of the 

 nerves. These muscles are bundles of fleshy fibres, 

 the contractions of which produce the various 

 movements of the animal body. Every extension of 

 the limbs, every elongation of parts, as well as the 

 shortenings and bindings, are occasioned by mus- 

 cular contraction. The number and direction of the 

 muscles in every animal are regulated by the mo- 

 tions it may have to execute, and vary with the 

 powers of motion peculiar to every species ; when 

 it is necessary that the motions should be per- 

 formed with vigour, the muscles are annexed to 

 hard parts, which are articulated generally one 

 upon another, and which may be considered as so 

 many levers. These parts in the vertebrated animals 

 are called bones, they are internal, and composed of 

 a gelatinous mass, interspersed with particles of 

 phosphate of lime. They are termed shells, crusts, 

 scales, in the mollusca, the crustaceous tribes, and 

 insects, where they are external and composed of 

 calcareous or horny substances, transuded between 

 the skin and the epidermis. 



The fleshy fibres are annexed to the hard part, 

 by means of other fibres of a gelatinous character, 



