Chap. 34.] Inftinfthe Attwrt, &V. 349 



has been called its vis infita. This power is mord 

 peculiar to life ; and though it may continue for a few 

 hours after death, yet it difappears much fooner than 

 the former. It was fuppofed by Haller to exift 

 wholly independent of the nerves, but this opinion has 

 neither been eftablimed nor refuted. The capacity of 

 inufcles to obey the influence of nerves, is called the 

 vis nervea. The power which we poflefs of calling 

 the mufcles into action by a voluntary effort, depends 

 on a relation eftablifhed by God, and not on the di- 

 "recting influence of the human foul, fince we have no 

 conception of the intimate ftructure either of the 

 nerves or mufcles. It is true, that by habit we acquire 

 a more exact command over our mufcles in moft in- 

 ftances, yet the operation of fucking, in which a va- 

 riety of mufcles operate in a complex manner, is 

 performed by a new-born infant; and the young of 

 many animals can walk immediately after birth. 



The extent of the contraction of a mufcle has been 

 limited by fome anatomifts to one-third of its length. 

 This ftatement, however, though it may be nearly juft 

 with refpedh to the greater number of mufcles, is by 

 no means true with refpect to all. The mufcular coat 

 of the bladder, for example, will admit of that organ, 

 containing a quart of fluid matter, without much in- 

 convenience, and at the fame time is fo contractile as 

 to be capable of expelling almoft eveiy drop in a very 

 fliort time. 



But the extenfive effect of mufcular contraction is 

 not owing only to the degree to which a mufcle can 

 fhorten itfelf, but alfo to the direction of its fibres. 

 Thus oblique mufcles produce a much more extenfive' 

 motion than thofe which are (trait, and this extenfive- 

 nefs of motion is proportioned to the obliquity of the 



mufcle. 



