Sect. XH. i. i. AND EXERTION. 45 



their approximation ; this invisible fomething is termed magaet- 

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

 animal mufcle, do not touch each other in the relaxed State 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 nan 

 ft&i luhere it does not exijl j for to a£l includes to zxijl ; and there- 

 fore the particles of the mufcular fibre (which in its State of re- 

 laxation are fuppofed not to touch) cannot affect 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 affix 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 illuftration or fimile to facil- 

 itate the conception of a difficult fubject. Let twenty very final! 

 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 poiltive, 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 lafl in the row, they will 

 all of them inftantly approach each other, and thus fhorten a line 

 that might connect them like a mufcular fibre. See Botanic 

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



The attractions of electricity or of magnetifm do not apply 

 philofophically to the illuftration of the contraction' of animal 

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

 portion inverfely as the diftance, but in mufcular motion there 

 appears no difference in velocity or Strength 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 aflimilated with 

 greater plaufibility to the attraction of cohefionor elasticity ; for 

 in bending a Steel fpring, as a fmall fword, a lefs force is re- 

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

 than the third ; the particles of fteel 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- 

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

 preflion of the particles on the concave fide than on the exten- 

 bon of them sn the convex fide j and by fij^pofing the elasticity 



of 



