PHYSIOLOGY. 97 



ference of force in different men, according to the quantity and 

 quality of their diet, and the reason why an English workman 

 may do double the business that a Scotchman can do in the 

 same time. There also appears here the effect of a full meal ; 

 for as the tension of one part depends upon that of every 

 other, the tension of the stomach necessarily has an effect upon 

 the whole system. It is upon this footing that evacuations 

 weaken the system ; that this depends upon the tension is evi- 

 dent from the fact, that evacuations from a particular part do 

 weaken that part more than the other parts. In this way, pro- 

 bably, the evacuation by purging weakens the tone of the in- 

 testines ; and in proportion as the evacuation is made by less 

 stimulus, this effect will be the greater. I conclude the con- 

 sideration of tension, which we have considered here simply as 

 giving vigour ; the explanation of the same tension, as a cause 

 of mobility, I must reserve till afterwards. 



" 4. Causes of excitement or collapse (CXXX.), applied 

 directly to the moving fibre itself. As in the case of sensibility 

 (LVL), the operation of these causes is sufficiently evident in 

 the diminution of tone by sedatives and narcotics ; but their ef- 

 fects as causes of increased tone are not so clearly perceived, 

 though a little more clearly than in the other case. In the oper- 

 ation of various stimulants, we perceive nothing more than their 

 producing a contraction, we do not perceive that they produce 

 any more vigour or irritability ; but this being given, they pro- 

 duce a contraction. Still, it is in general probable, that there 

 are powers of excitement as well as of collapse, and that among 

 the stimulant powers there are some powers of excitement which 

 act in regard to the moving fibre. But however it may be with 

 the stimulants, there are other powers which increase the tone of 

 muscular fibres ; such are those we call astringents and tonics. 



u To this head of causes of difference in vigour, I must refer 

 the power of exercise also. The effects of this in giving strength 

 and vigour in certain conditions, are pointed out in CXIV. ; 

 but these effects are attended with a change in the muscle itself, 

 with a manifest increase of bulk in such muscles; exercise, 

 therefore, acts directly upon the muscular fibre, as well as upon 

 the whole system ; but how it acts Upon the muscular fibre is 

 not clear. We can observe that it gives more facility in action, 

 VOL. T. G 



