STRUCTURE OF MUSCLES. 



two classes of exciting causes in ordinary circumstances. In 

 some cases, it is true, \\ e can acquire by habit a controul 

 even over the involuntary muscles; and the muscles which 

 are voluntary in one animal, may be involuntary in another. 

 We may, however, add, as peculiar characters by which 

 they may be distinguished, that the involuntary muscles 

 are more durable in their action, more easily excited, and 

 retain, even when separated from the body, their facility of 

 irritability for a great length of time ; while the volunta- 

 ry muscles become fatigued by continued action, and 

 require intervals of rest to recruit their exhausted ener- 

 gies. 



When the fibres of a muscle are excited to action by any 

 irritating cause, a simultaneous movement is performed. 

 They become shorter, more rigid, and in many cases ap- 

 pear somewhat angular. In consequence of this contrac- 

 tion, the two extremities of the muscle approach, bringing 

 along with them the parts to which they are attached. In 

 what manner the particles of the muscular fibre arrange 

 themselves during contraction, anatomists have been unable 

 to discover. That they approach each other, is obvious ; 

 and that their cohesive power suddenly increases, appears 

 evident, from the small force which prevents a muscle from 

 contracting, compared with the prodigious resistance which 

 it offers when contracted. Dr WOLLASTON is of opinion, 

 that each muscular effort, apparently single, consists in re- 

 ality, of a great number of contractions, repeated at ex- 

 tremely short intervals ; so short, indeed, that the interme- 

 diate relaxation cannot be visible, unless prolonged be- 

 yond the usual limits, by a state of partial or general debi- 

 lity. He arrived at this conclusion, by attending to the 

 sounds perceived in the ear upon the insertion of the tip of 

 the finger. It is probable, however, that these sounds are 



