PHYSIOLOGY. 89 



usual force, and such contractions are often repeated, they in a 

 short time become uneasy and weaker ; and though contractions 

 are not exerted with unusual force, if they are often repeated, 

 and for a long time, without an interval of rest, they also become 

 uneasy and weaker. " There is here, perhaps, a little inac- 

 curacy ; the unusual force may be referred to the cause of con- 

 vulsion ; but I mean here, that if the contractions of muscles, 

 by their ordinary causes, are exerted with a little more than 

 ordinary force, though that is measured and directed by the 

 will, if such are often repeated, under these circumstances they 

 in a short time become uneasy ; and though the contractions 

 are made by natural causes, if they be often repeated, and for 

 a long time without an interval of rest, they become uneasy and 

 weaker, they give the sense of lassitude, and we cannot continue 

 to exert the same force with which we set out in such exercise. 

 Nothing is a more common observation than that the muscles are 

 exposed to an uneasy sensation which we call fatigue, and mani- 

 festly become weaker : so men, used to labour for ten or twelve 

 hours of the day, by habit are able to do with ease what would 

 be fatiguing to us. But even these men, if they are obliged 

 one day to protract their labour for more than that time, 

 become fatigued ; and if they continue to urge on their labour, 

 they will fall down, perhaps, entirely powerless, or perhaps they 

 will fall into a deliquium altogether. The former part of the 

 proposition will be as obvious, viz. that though the exercise is not 

 continued for any unusual length of time, if it is for some time 

 carried on with unusual force, the same fatigue, lassitude, and 

 debility consequently ensue. I presume that it would be the 

 case with most of us, that were we to attempt the work of a 

 labourer in digging ground, we not only could not continue it 

 for any length of time, but as we are accustomed to exercise 

 our motions with a given velocity and force, the fatigue would 

 be sooner adduced by our excess in this respect. I wish I could 

 here account for this, and explain upon what it depends ; but till 

 we know much more of the nature of muscular motion, and of the 

 connexions of the system, how the several parts co-operate, and 

 how far the animal power concurs upon this occasion, we are en- 

 tirely at a loss to say any thing of this matter ; but I shall have 



