SECT. XII. i. i. AND EXERTION. 45 



their approximation ; this invisible fomething is termed magnet- 

 ifm. In the fame manner, if the particles, which compofe an 

 animal mufcle, do not touch each other in the relaxed ftate of 

 the mufcle, and are brought into contact during the contraction 

 of the mufcle, it is reafonable to conclude, that fome other 

 agent is the caufe of this new approximation. For nothing can 

 atf> where it does not exifl ; for to aft Includes to exift ; and there- 

 fore the particles of the mufcular fibre (which in its (late of re 

 laxation are fuppofed not to touch) cannot effect each other 

 without the influence of fome intermediate agent ; this agent is 

 here termed the fpirit of animation, or fenforial power, but may 

 with equal propriety be termed the power, which caufes con- 

 traction ; or may be called by any other name, which the reader 

 may choofe to aifix to it. 



The contraction of a mufcular fibre may be compared to the 

 following electric experiment, which is here mentioned not as 

 a philofophical analogy, but as an illuflration or fimile to facil- 

 itate the conception of a difficult fubject % Let twenty very fmall 

 Leyden phials properly coated be hung in a row by fine filk 

 threads at a fmall diftance from each other ; let the internal 

 charge of one phial be pofitive, and of the other negative alter- 

 nately, if a communication be made from the internal furface of 

 the firft to the external furface of the laft in the row, they will 

 all of them inllantly approach each other, and thus (horten a line 

 that might connect them like a mufcular fibre. See Botanic 

 Garden, P. I. Canto 1. 1. 202. note on Gymnotus. 



The attractions of electricity or of magnetifm do not apply 

 philofophically to the illufiration of the contraction of animal 

 fibres, fince the force oi thofe attractions increafes in fome pro- 

 portion inverfcly as the diftance, but in mufcular motion there 

 appears no difference in velocity or ftrength during the begin- 

 ning or end of the contraction, but what may be clearly afcribed 

 to the varying mechanic advantage in the approximation of one 

 bone to another. Nor can mufcular motion be affimilated with 

 greater plaufibility to the attraction of cohefion or elafticity ; for 

 in bending a fleel fpring, as a fmall fvvord, a lefs force is re- 

 quired to bend it the firil inch than the fecond ; and the fecond 

 than the third j the particles of fleel on the convex fide of the 

 bent fpring endeavouring to reftore themfelves more powerfully 

 the further they are drawn from each other. See Botanic Gar- 

 den, P. I. addit. Note XVIII. 



I am aware that this may be explained another way, by fup~ 

 pofing the elafticity of the fpring to depend more on the com- 

 prefiion of the particles on die concave iide than on the trxten- 

 fion of them on the convex fulc \ and by fuppofing the elaiiicity 



of ' 



