90 PHYSIOLOGY. 



occasion to recur to it, when we have discussed another law, 

 which is of more importance."" 



CXIV. Within certain bounds, with respect to force, frequen- 

 cy, and duration, the contraction of muscles, by being repeated, 

 is performed with more facility and force. " There is not a man 

 who has not bounds of that kind ; he can repeat his actions for a 

 certain number of tunes, and for a certain length of time, without 

 any fatigue or debility ; and when they go no farther, they have 

 a contrary effect, the contraction is performed with more facility 

 and force. Hardly any body has entered upon the learning of 

 a new exercise, who does not know that the first attempts are 

 what we call awkward. There is a concurrence of various mus- 

 cles and members, that do not readily at first accommodate 

 themselves to perform the motions exactly. Only by repeti- 

 tion we acquire the command over certain muscles, and are 

 enabled to unite and accommodate various motions to one and 

 the same action ; and there is nobody who has not observed, 

 that whether the motions are more simple or complex, we are 

 constantly improving in the facility of moving. Thus, in the 

 management of a musical instrument, we find that our fingers 

 have been associated in some motions only : we must break 

 these associations, and they disappear by repetition ; and the 

 agility and surprising motions thus acquired, astonish every 

 body who has not had the same experience. 



" I have an observation of more consequence to make, viz. that 

 somewhat of this kind takes place with regard to involuntary 

 motions, under the power of stimuli of different kinds ; these 

 also, by repetition, are performed with more facility, so that they 

 can be excited to the same contractions with the application, per- 

 haps, of a tenth part of the force. If the exhibition of an emetic, 

 or any other medicine, whose effect is to be muscular contraction, 

 is frequently repeated, a less and less dose is found to be ne- 

 cessary. I have known a person who thought it necessary to 

 vomit very frequently sometimes every day, and at least twice 

 a week. At first he required three grains of tartar emetic, 

 but at length half a grain would not only excite full vomiting, 

 but was liable to throw him into cramps and spasms ; and if by 

 any accidental interruption, he imagined that a larger dose 



