382 
POPULAR SCIENCE REVIEW. 
Still further, the argument of the velocity with which im- 
pressions are conveyed from the centres of the nervous system 
to the muscles, or from the external surfaces back to the centres 
— an argument which in our times has been advanced as if it 
were new in science — was studied by these earlier physiologists 
who urged that if the nerves are constantly filled or charged with 
fluid, an impulse given to that fluid at the brain may be sud- 
denly communicated to the most distant organ, or the reverse, 
although the velocity of the fluid be itself very small. 
The modern reader will gather from the above that there 
was great simplicity, great beauty, great force, in the olden 
theory of a nervous fluid ; and if I could invite him to follow 
me into the subject of structure of nervous matter he might 
perchance be more convinced that the theory was in a sense 
true and unassailable. He would certainly wonder why so 
little is known, at this day, about so important a speculation. 
His wonder would, moreover, be most reasonable, for the 
theory, while it has never been successfully assailed, never 
been injured a jot by anything that has been said about it, has 
merely been let drop and forgotten, hidden for a time by the 
brilliancy of another theory — now well-nigh blazed out — I mean 
the electrical theory promulgated by Gralvani. 
The evidence in favour of the existence of an elastic medium 
pervading the nervous matter and capable of being influenced 
by simple pressure is all-convincing. When we press a nerve 
firmly we act on something that is as distinctly under the 
influence of the pressure as when we press upon a vein and by 
that means influence the current of blood within the vein. 
When we freeze a nerve we stop its function altogether, we 
make it almost like metal in its appearance and physical 
character, and we can then divide it without communicating the 
faintest scintillation of sensation to the brain.* In this case 
the cold has acted like pressure ; it has either condensed the 
nervous matter, and has, by the contraction induced, driven out 
some agent the presence of which was necessary for the per- 
formance of function, or it has condensed the agent itself to- 
gether with the nerve, so that the condition for sensation is 
suspended. When we divide a nerve we break a connection, 
we divide a structure which must be absolutely perfect for 
conveyance of motion, and with our best skill we cannot secure 
that the connection shall be ever again rendered perfect. 
Each one of these experimental facts suggests that there 
exists in the nerve an actual material mobile agent, a something 
* This fact is well illustrated in the operation of nerving the horse, under 
ether spray. The nerve, when exposed, is found superficially frozen, and 
when frozen more deeply may be cut as if it were a soft metallic wire, dead 
to the knife as in the nerveless horny hoof below. 
