Chap! 34.] Po-toer of the Will. 347 



general red *, fibres, combined together in bundles by 

 cellular fubftance. Such, therefore,- being the ftruc- 

 ture of mufcles, little credit appears to be due to the 

 fuppofition, that mufcular contraction depends on an 

 influx of blood or any other fluid into minute blad- 

 ders or cells ; and it muft remain undetermined, 

 whether the ultimate moving fibres are tubular or 

 folid ; whether they confift of chains of rhomboidal 

 veffels, as has been imagined by fome phyfiologifts, 

 or whether, as others have thought, they contain a 

 kind of down or woolly fubftance. 



When mufcles are thrown into action, they become 

 fhorter, broader, and more denfc, or folid, to the 

 touch ; their bulk does not feem to be on the whole 

 increafed, nor are they found to be of a paler colour. 



Mufcles never act but from fome exciting caule ; 

 of thefe one of the mod frequent and curious is 

 volition, by which every day's experience teaches us 

 we have the power of throwing the greater number 

 of our mufcles into action. Over fome of our 

 mufcles, however, as thofe of the interlines, and the 

 heart, the will has no influence, and thefe are there- 

 fore called mufcles of involuntary motion. What 

 is the nature of the influence which the will exerts 

 over mufcles, we can never hope to difcover ; but 

 it is of importance to remark, that the nerves are the 

 organs by which this influence is exerted ; for the 

 nerve leading to any particular limb being divided, 

 we are no longer able to move that limb at our 

 pleafure. 



Befides, however, being influenced by the will, 

 mufcles are thrown into action by feveral other caufes, 



* The colour proceeds from the blood, which they contain in 

 minute veflels. , 



fuch 



