11(5 PHYSIOLOGY. 



cessary to the action of muscular fibres. 2. That custom asso- 

 ciates motions with sensations, which are not otherwise their 

 causes ; so that the renewal of the sensation, or of its idea, re- 

 news also the motion. 3. That custom associates different mo- 

 tions, so that they cannot be separately performed, though not 

 originally nor necessarily connected ; " as is observed in the 

 childish play of rubbing the body with one hand and striking it 

 with the other." 4. That custom determines the degree of force 

 and velocity with which motions can be performed. " Every 

 action, especially where it is a matter of choice, we perform 

 slower or faster ; thus, every man, in writing, falls into a man- 

 ner (to use the language of painters) ; and such a degree of 

 force and velocity comes to be a law, so that we can no more 

 perform the same motion with a different velocity than we can 

 work with tools of different weights." 5. That custom deter- 

 mines the order of succession in associated motions, and the ve- 

 locity with which they shall succeed one another. " If a per- 

 son has got a passage by heart, and, after a great interval of 

 time, his memory is not so ready with regard to it, he may re- 

 cover it, by having given to him a word or two at the begin- 

 ning : he will then go on, but losing a little, must return to the 

 beginning, and repeat the whole together. As the velocity of a 

 motion is determined by custom, so the velocity with which mo- 

 tions succeed one another also is : a boy to repeat his lesson 

 must do it as when at school ; if we insist on his doing it more 

 slowly, he cannot do it at all.*" 6. That custom establishes the 

 periodical return of certain sensations and motions, not origin- 

 ally necessary to the economy. 7- That custom fixes an exact 

 period for the return of certain sensations and motions, which, 

 by the laws of the economy, are disposed to return at intervals 

 otherwise undetermined. These laws, which may be establish- 

 ed by custom, are, many of them, with difficulty avoided, 

 " though, for many different purposes, it is desirable to avoid 

 them. We thereby preserve a greater latitude of health : at 

 the same time, such is the constitution of our economy, exposed 

 to vicissitudes, and to external things favouring these, and such 

 is the uniformity in the business of life, that we must be sub- 

 ject to habits. They increase with the advance of life ; but it is 

 desirable to preserve some latitude as long as we can ; for, when 





