262 



ON MUSCULAR MOTION. 



A contra£lcd 

 inufcle is lefs 

 fenfible. 



Moral caufes in- 

 fluence the 

 mufcles in the 

 human fpecies. 



Mental as well 

 as mufcular ac- 

 tions may by 

 habit become 

 automatic) 



and then give 

 little fatigue. 



Voluntary and 



involuntary 



mufcles. 



When mufcles are vigoroufly con traced, ihe\r fenfibJIity 

 to pain is nearly dellroyed ; this means is employed by jug- 

 glers for the purpofe of fufFering pins? (o be thriift into the 

 calf of the' leg, and other mufcular parts with impunity : it is 

 indead reafonable to expe(5>, a priori, that the fenfalion, and 

 the voluntary influence, cannot pafs along the nerves at the 

 fame time *: 



In addition to the iiifluences already (?nDmeratedi the human 

 mufcles are fufceptibie of changes from extraordinary occur- 

 rences of fenfible impreffions. Long continued attention to 

 interefting vifible obje6ls, or to audible fenfations, are known 

 toexhauft the mufcular flrenglh : intenfe thought and anxiety, 

 W'Caken the mufcular power'^, and the paffions of grief and 

 fear prodnce the fame effect fuddenly : whilft the contrary 

 feelings, fiich'as the profped of immediate enjoyment, or 

 moderate hilarity, give more than ordinary vigour. 



It is a very remarkable fa6t in the hiflory of animal nature, 

 that the menial operations may become almoft automatic, and, 

 under fuch habit, be kept in adion, without any interval of 

 reft, far beyond the time which the ordinary flate of health 

 permits, as in the examples of certain maniacs, who are en- 

 abled without any inconvenience, to exert both mind and 

 body for many days inceflantiy. The habits of particular 

 modes of labour and exercife are foon acqaired, after which; 

 the actions become automatic, demand little attention, ceafe 

 to be irkfome, and are effeded with little fatigue : by this 

 happy provifion of natrfre, the habit of induftry becomes^ 

 foprce of pleafure, and the fame appears to be extended to 

 the docile animals which co-operate with man in his labours. 



Three clalTos of mufcles are found in the more complicated 

 animals. Thofe which are conftantly governed by the will, 

 or direding power of the miridi are called voliJiitary mufcles. 

 Another clafs, which operate without the confcioufnefs of the 

 mind, are denominated involuntary; and a mixed kind occur 

 in the example of refpiratory mufcles, which are governed by 

 the will to a limited extent ; neverthelefs the exigencies of the 



• I have often obftrved that a fmall ele6^ric flioek may be re- 

 ceived without pain through the mufcles of the fore arm ; but I 

 imagined it to be owing to the want of power in fuch a fhock to 

 increafe the contra^ion. — N, 



animal 



