ON MUSCULAR MOTION, 195 



jdn^ions, called Ganglions, and which are compofed of a lefsGanglions. 

 proportion oF the medullary (ubftance, and their texture is 

 firmer than that of ordinary nerves* 



The terminal extremities of nerves have been tifuatly con- The extfcmitlci 

 fidered of unlimited extenfion ; by accurate dilFedion hov<'-°^.""7" "p^^"' 

 ever, and the aid of magnifying glafles, the extreme fibrils of termination* 

 nerves are eafily traced as far as their fenfible proper ties* and 

 their continuity extends. The fibrils ceafe to be fabdivided 

 whiift perfectly vifible to the naked eye, in the voluntary 

 mufcles of large animals, and the fpaces they occupy upon 

 luperficies where they feem to end, leave a remarkable excefs 

 of parts unoccupied by thole fibrils. Tiie extreme fibrils of 

 nerves lofe their opacity, the medullary fubftance appears foft 

 and tranfparent, the enveloping membrane becomes pellucid, 

 and the whole fibril is deftitute of the tenacity necefiary to 

 preferve its own diftinfinefs ; it feems to be diftbfed and 

 mingled with the fubftances in which it ends. Thus the ulti- 

 mate terminations of nerves for volition, and ordinary fenfation, 

 appear to be in the reticular membrane, the common covering 

 of all the diflJerent fubftances in an animal body, and the con- 

 necting medium of all diffimilar parts* 



By this fimple difpofition, the medullary fubftance of nerveDiffufionof nef* 

 i^ fpread through all organized, fenfible, or motive parts/°"^ matter, 

 forming a continuity which is probably the occafion of fvm- 

 pathy. Peculiar nerves, fuch as the firft and fecond pairSj, 

 and the portio mollis of the feventh, terminate in an expanfe 

 of medullary fubftance which combines with other parts and 

 membranes, ftill keeping the fenfible excefs of the peculiar 

 medullary matter. 



The peculiar fubftance of nerves muft in time become inef-Reftoration or 

 ficient ; and^ as it is liable to injuries, the powers of reftoration,'^®^^^ ^^ nciyis^ 

 and repair, are extended to that material. The re-union of 

 nerves after their divifion, and the reprodudion after part of a 

 nerve has been cut away, have been eftablilhed by decifive 

 experiments. Whether there is any new medullary fubftance 

 employed to fill up the break, and, if fo^ whether the new 

 fubftance be generated at the part, or protruded along the 

 nervous theca from the brain, are points undetermined ; the 

 hiftory of the formation of a iietus, the ftruclure of certain 

 monfters, and the organisation of fimple animals, all feem to 



O 2 favour 



