280 Extreme Tenuity of Nerves. [Book IX. 



blifhed, becaufe a decufiation in fome parts is by no 

 means a proof that it obtains univerfally; and though 

 there are inftances of injuries of the head, which have 

 produced a palfy of the oppofite fide, there are others 

 in which the injury and palfy were both on t(ie fame 

 fide. 



Nerves are compofed of threads of the fmallnefs ot 

 which we have probably no adequate idea. To afiift 

 us in forming one, we muft confider how uniformly 

 nerves are distributed to even the moft minute fibre of 

 the body, and yet were they all conjoined, they would 

 not make a cord of an inch diameter. It is deuuced 

 from actual obfervation, that each fibre in the retina of 

 the eye, or expanded optic nerve, cannot exceed in 

 diameter the thirty-two thoufand four hundredth part 

 of a hair. 



Different nerves in their courfe often meet together, 

 and form oblong reddifh maffcs, called ganglions, 

 larger than the nerves which form them, and alfo of a 

 firmer corfiftence. Within the ganglions the fibres 

 of the nerves feem to be thoroughly mixed, and to 

 approach more nearly to the nature of medullary mat- 

 ter. By fome phyfiologifts ganglions are fuppofed to 

 be frnall brains, whence the nerves acquire new power 

 and energy. Others, obferving that the nerves which 

 fupply the mufcles of involuntary motion, as thofe of 

 the he;irt and inteftines, are particularly fupplied with 

 ganglions, have confidered them as defigned to inter- 

 cept the operation of the will. Their real ufe is un- 

 known to us, but from whatever caufe it may happen, 

 the nerves which proceed from a ganglion are rather 

 larger than the fum of thofe which form it. ' 



Several nerves frequently meet together, and by nu- 

 merous junctions produce an appearance firriilar to that 

 of net-work, and this is called a plexus. 



Nervous 



