34G 
PHYSIOLOGY. 
mining towards the mind impreflions from without, ano¬ 
ther of tranfmitting an impreflion derived from the latter 
fource to other parts of the body, the mufcles or veffels. 
But recently the well-known experiments of Bell, repeated 
by Magendie, and multiplied to a barbarous and unnecef- 
f'ary extent by many dabblers in phyfiology, have led to 
the conclufion, that thefe two functions are performed 
by different nerves; fo that, in a part poflefling both 
motion and fenfation, one fet of nerves caufes it to move, 
another fet to feel; and, if (as in the tongue) there be 
two fpecies of fenfation in one organ, each of thefe alfo 
has a diftinCt nervous apparatus. 
A remarkable contrail is obfervable between the 
motive and fenfitive powers of nerves. Let us confider the 
former. From all that we can learn of the nature of this 
agency in nerves, it feems to confift in the aCtual and 
fubftantial tranfmiflion of a fluid analogous to the eleCtric 
(if the eleCtric be a fluid) to the parts to be moved : the 
motion itfelf requiring certain proportions of oxygen and 
heat to its production, feems to fltow that eleCtricity is 
fomehow employed in this motion, feeing how neceflary 
both oxygen and heat are to eleCtrical phenomena: laffly, 
the eifeCl of an electrical Ihock, not only when applied to 
the mufcles themfelves, but even when aCting on the 
remoter ends of their nerves, goes toeftablifh the identity 
of the nervous and eleCtric fluid. 
This opinion, often entertained by philofophers, has 
been ftrengthened by the inferences drawn from experi¬ 
ments on fecretion. The refearches of Brodie, Philip, 
and others, have Ihown, that fecretion in thofe important 
organs, the ftomach and lungs, ceafes, or nearly ceafes, 
with the diviflon of their nerves; but moreover Philip 
has difcovered, that, after this diviflon, eleCtricity will 
reftore, fo long as it is applied, the powers of the fecer- 
nents. From thefe faCls the conclulion has been made, 
that the nervous and eleClrical fluids are precifely identi¬ 
cal ; and, when we confider how mighty are the changes 
induced by eleCtricity in ail nature, we fhould be inclined 
to adopt this idea. It feems, too, a very Ample view of 
the fubjeCt, to confider eleCtric fluid the fecretion of the 
brain; to regard this fluid as now operating by a fliock of 
the mufcle, and caufing us to move, now aCting on the 
fluids palling through capillaries, and fo altering thofe 
fluids (and it feems to produce as great changes out of 
the body) as to produce from them every fecretion, every 
ffruflure. 
We at firlt took this view of the fubjeCl ourfelves; but 
it is not the correCt one. We have before faid, when 
fpeaking of the fecerning tube, that in vegetables nearly 
as great changes were induced as in animals; yet this 
mighty galvanic battery does not exift in the former, nor 
indeed in many of the latter. When perfons have talked 
of the mighty power of eleCtricity, they have forgotten 
that this power may, even in inanimate nature, be likened 
rather to a Jiimulus which excites the properties of 
matter, than as a foie and fuperior agent; for, if eleCtri¬ 
city aCted independently of the properties of bodies, its 
effeCts would be always uniform, whereas they are various. 
Now, this being allowed, we fay, that the minute veffels 
which form the fubftance of every part of animals are the 
true operators which induce the various changes their 
contents undergo ; and that, in vegetables, thefe inde¬ 
pendent properties feem to be inherent ; growing as 
they grow ; but that, in animals, they require the tranf¬ 
miflion of nervous or eleCtric fluid, though merely for the 
maintenance of thofe powers : and accordingly we find in 
the human body, that, where fecretions are Ample, few or 
no nerves fuffice to keep up the powers of the lecernen-ts ; 
but that, in the more important one, the fupply of nerves 
is abundant. With refpeCt to the quellion whether the 
eleCtric and nervous fluids are identical, it cannot be 
eafily determined, nor is it of great confequence. There 
certainly are ftrong arguments in fupport of the affirma¬ 
tive. 
The function of fe.vjation is entirely diftinCt from that 
of motion. It is not in the lead degree analogous to 
eleCtricity, nor indeed is it analogous to any thing we 
know. Motion is of one kind; fenfation is almoft infi¬ 
nite. Who fliall count the number of impreflions the 
optic or the olfaCtory nerve can take ? no one : they are 
as many as the objeCts in nature. Phyfiologifts have, 
notwithftanding this, fancied fome fimilitude between 
thefe two. Bichat has even gone fo far as to call the 
fpecific attraction of the fecernent tube its Jenfibility. 
In defcribing the refpeCtive parts of the nervous fyftem 
to which fenfation and motion are attached, we mult firft 
give a complete analyfis of the labours of Mr. Charles 
Bell, that diftinguifhed anatomift having firft eftablilhed 
that thefe functions belonged to feparate nerves. 
This author, then, confiders the nerves as diftin- 
guiflied, from their functions, into two daffies; the one 
compofing what he calls the original or fymmetrical 
fyftem, and the other the fuperadded or irregular fyftem. 
The former are more exprefsly provided for the purpofes 
of fenfation and locomotion. In animals where thefe 
functions are not complicated with thofe of circulation 
and refpiration by central organs, thefe nerves are very 
Ample, confifting merely of two cords, running the whole 
length of the body, and giving off lateral branches to the 
feveral divifions of which their annulated frame is com- 
pofed. This is the cafe with infeCts, and with molt of 
the vermes. As we afcend to the higher orders of ani¬ 
mals, we find a greater complication of functions, and a 
greater intricacy of nervous connexions arifing from the 
necefiity of eftablilhing extenfive links of affociation 
between the organs that perform thefe additional func¬ 
tions. Hence the fecond clafs of nerves is provided, 
which crofling the former in a variety of directions, and 
occafionally uniting with them, gives rife to the intricacy, 
and apparent confufion, in which the anatomy of the 
nervous fyftem has hitherto been involved. The nerves 
belonging to the firft clafs may be diftinguifhed in the 
human body as forming the original fyftem, if abftraCtion 
be made of all the fuperadded nerves. The nerves of 
the fpine, the tenth or fub-occipital nerve, and the fifth 
or trigeminus of the fyftem of Willis, conftitute this 
fyftem. All thefe nerves agree in the following effential 
circumftances : they have all double origins; they have 
all ganglia on one of their roots ; they go out laterally 
to certain divifions of the body; they do not interfere to 
unite the divifions of the frame; they are all mufcular 
nerves, ordering the voluntary motions of the frame; 
they are all exquifitely fenfible, and the fource of the 
common fenfibility of the furfaces of the body. When 
accurately reprefented on paper, they are feen to pervade 
every part; no part is without them ; and yet they are 
fymmetrical and Ample, as the nerves of the lower animals. 
On the other hand, the nerves which conned the internal 
organs of refpiration with the fenfibilities of remote 
parts, and with the refpiratory mufcles, are diftinguifhed 
from the former by their not arifing from double roots, 
and having no ganglia on their origins; they come cff 
from the medulla oblongata and upper part of the fpinal 
marrow; and from this origin they diverge to thofe 
feveral remote parts of the frame which are combined in 
the motions of refpiration. If the nerves be expofed in a 
living animal, thofe of the former clafs exhibit the high- 
eft degree of fenfibility ; while, on the contrary, thofe of 
the fecond are comparatively fo little fenfible, as to be 
immediately diftinguifhed ; infomuch that the quiefcence 
of the animal fuggefts a doubt whether they be fenfible 
in any degree whatever. If the fifth pair, and the portio 
dura of the feventh, be both expofed on the face of a 
living animal, there will not remain the flighted: doubt 
in the mind of.the experimenter, which of thefe nerves 
bellows fenfibility. If the nerve of the firft clafs be 
divided, the fkin and cellular fubftance are deprived of 
fenfibility; but the diviflon of nerves not belonging to- 
this clafs does not at all deprive the parts of their fenfi¬ 
bility to external impreflion. There is alio a wide di- 
3 ftinCtiou 
