m
74
A
O B S E R V A T I O N S ON THE
C H A P T E R XXIIL
Of the Nature of the Energy of the Nerves.
F T E R examining the ftru£lure of the nerves, we are led to confider
the nature of the nervous energy.
Moil authors have fuppofed that the nerves are tubes or dufts conveying
a fluid fecreted in the brain, cerebellum, and fpinal marrow. But, of late
years, feveral ingenious phyfiologifts have contended, that a fecreted fluid
was too inert for fer\'ing the ofiices perfonned by the nerves, and, therefore,
fuppofed that they conduced a fluid the fame as, or funilar to, the
elcdlricai fluid.
Two arguments chiefly feem to have led them to this conclufion.
The nervous energy appeared to them to be moved with prodigious
velocity. Thus, Dr Haller obferVes how often a mufcle of any part
could z£t in a minute; and fuppofing that, previous to every contradion,
the nervous fluid moved from the brain to that part, has attempted to calculate
its velocity *.
But if the nerves are conftantly filled or charged with their fluid or energy,
which, from our inftantly perceiving injury done to every the moll
remote part, we have reafon to fuppofc is the cafe, an impulfe given to
that fluid at the brain may be fuddenly communicated to the mofl: difl:ant
organ, although the velocity of the fluid be very fmall. Nay, in fa£t, we
find,
• Haller, El. Phyf. L. x. and xl. L. x. p. 373. Ita invenio, fummam tamen celeritatem effe mufcularis
liquidi, ut non minus quam 9000 pedes in rainuto prinio percurrat.
N E R V O U S SYSTEM. 75
find, after cutting through the nerve of a mufclc, that, by irritating the
nerve, repeated motions can be performed ; whereas, by Dr Hallcr's theory
of great velocity from end to end of the nerve, as thé nerve now wants
fupply from the refervoir in the brain, the fluid fliould be exhauiled by a
fingle effort of the mufcle. Such an argument as this no more proves tiiat
the nervous fluid moves with great velocity, than our letting out, in tlie
fpace of a minute, a hundred fucceflTive drops of water from the end of a
pipe, a mile difl:ant from the refervoir which filled it, would prove that
the water was moving in that pipe at the rate of a hundred miles every
The other principal argument is more dired, and has been thought veiy
conclufive, to -wit, that fome animals, as the torpedo and gymnotus electricus,
have the power of giving an ele£lrical ihock, and that, on diifeding
them, a piece of machinery, proper to them, is difcovered, in which large
and numerous nerves terminate.
But, perhaps, all we can conclude from thefe fads is, that the nerves
enable this machinery to perform its proper office of coUefling the eledrical
fluid, but -without diredly furnifliing to it any of that fluid. Jufl: as we,
by rubbing a glafs tube, excite eledricity without there being any reafon
to fufped that the eledrical matter is, in particular, derived from the nerves
of our hand, fince the eledricity could be as readily excited by the hand of
a dead man, as by that of a living, rubbed againfl: the glafs with the fame
force.
We feem, therefore, far from pofTcflling pofitive arguments that the
nerves operate by the medium of an eledrical fluid, efpecially if we add,
that, after cutting a ner\'e acrofs, and again bringing its parts contiguous,
its offices cannot be immediately reftored. Nay, I have found in frogs,
that, although the divided parts grow together again, the influence of the
nerve beyond the inciflon is generally not recovered. On the other hand,
the