on muscular Motion. 
9 
substance throughout the canal of the vertebrae. Another 
class of muscles, which are not subject to the will, are supplied 
by peculiar nerves ; they are much smaller, in proportion to 
the bulk of the parts on which they are distributed, than those 
of the voluntary muscles ; they contain less of the white 
opaque medullary substance than the other nerves, and unite 
their fibrils, forming numerous anastomoses with all the other 
nerves of the body, excepting those appropriated to the organs 
of the senses. There are enlargements at several of these 
junctions, called Ganglions, and which are composed of a less 
proportion of the medullary substance, and their texture is 
firmer than that of ordinary nerves. 
The terminal extremities of nerves have been usually con- 
sidered of unlimited extension ; by accurate dissection how- 
ever, and the aid of magnifying glasses, the extreme fibrils of 
nerves are easily traced as far as their sensible properties, and 
their continuity extends. The fibrils cease to be subdivided 
whilst perfectly visible to the naked eye, in the voluntary 
muscles of large animals, and the spaces they occupy upon 
superficies where they seem to end, leave a remarkable excess 
of parts unoccupied by those fibrils. The extreme fibrils of 
nerves lose their opacity, the medullary substance appears soft 
and transparent, the enveloping membrane becomes pellucid, 
and the whole fibril is destitute of the tenacity necessary to 
preserve its own distinctness ; it seems to be diffused and 
mingled with the substances in which it ends. Thus the ulti- 
mate terminations of nerves for volition, and ordinary sensation, 
appear to be in the reticular membrane, the common covering 
of all the different substances in an animal body, and the con- 
necting medium of all dissimilar parts. 
MDCCCV. C 
