PHYSIOLOGY. 
nervous influence, ift, it takes place in animals in 
which there is no appearance of nerves”; idly, it is very 
remarkable in fome fpecies of vegetables; 3dly, it 
appears that the fibrine of the blood, which vve can 
fcarceiy fnppofe to be affedled by the nervous power, 
when taken out of the body, is ftill fufceptible of irrita¬ 
tion. Hence we mull either conclude, that the irritabi¬ 
lity of living mufcles, and of the fuperior animals, is dif¬ 
ferent from that of the fibrine, of polypes and plants; or, 
if we admit that nervous influence is elfential to irrita¬ 
bility, vve mult alfo allow that this influence defcends to 
the latter clafs of organized bodies. 
Though hitherto futile, yet, as they may lead future 
obfervers to more important knowledge, it is proper to 
infert alfo the opinions of fome chemical philolophers on 
the fubjedt of irritability. The firlt of thefe is that of 
Girtanner, who confidered oxygen as the principle of 
irritability. The arguments on which he founded this 
opinion are the following. 1. The irritability of orga¬ 
nized bodies is always in a diredl ratio to the quantity of 
oxygen they contain. 2. Every thing that augments the 
quantity of oxygen in organized bodies augments at the 
fame time their irritability. 3. Every thing that dimi- 
nillies the quantity of oxygen diminilhes likevvife their 
irritability. 
He diftinguilhes the organized fibre by three different 
Hates: 1. A Hate of health, or the tone of the fibre, in 
which the oxygen exifts in its proper quantity. 2. A 
Hate of accumulation , in which the fibre is overcharged 
with the oxygen, or irritable principle. 3. A Hate of 
exhanftion, in which the fibre is more or lefs deprived of it. 
He likewife arranges the fubftances, that are capable of 
coining into contadl with the irritable fibre, into three 
claffes. The firlt comprehends thofe fubftances that have 
the fame degree of affinity for the irritable principle, or 
oocygen, as the organized fibre itfelf; hence the fubftances 
produce no effedt upon it. The fecond comprehends 
thofe fubftances that have a lefs degree of affinity for oxy¬ 
gen than the organized fibre has; hence thefe, when they 
come into contadl with it, furcharge it with oxygen, and 
produce a Hate of accumulation. They are called negative 
ftimuli. The third comprehends fubftances for which 
oxygen has a greater affinity than it has for the organized 
fibre. Thefe, therefore, deprive the fibre of its oxygen, 
and produce a Hate of exhaujlion. They are called pofi- 
tive ftimuli. 
In anfwer to this dodlrine, it has been obferved, that, 
if oxygen were fo elfential to irritability as is fuppofed in 
Girtanner’s pofitions, thofe animals which refpire moll 
oxygen lhould poffefs moll irritability, and thofe which 
are capable of living for a long time in deoxygenated air 
lhould have their irritability very low. Now, the reverfe 
of this is found to take place: the mufcular fibres of 
birds, which refpire more oxygen than moll other ani¬ 
mals, poffefs but little irritability; wdiile reptiles and 
worms, which can live for a long time without oxygen, 
are univerfally and ftrongly irritable. 
Humboldt confiders the galvanic fluid as the fource of 
nervous power, and the primary caufe of irritability. He 
lays down three principles as neceffary to excite irritabi¬ 
lity ; viz. 1. Oxygen, which forms combinations with 
different acidifiable bafes. a. The acidifiable bafes (car¬ 
bon, hydrogen, azote, and phofphorus) of the fibre, with 
which the oxygen may combine. 3. The galvanic fluid. 
The galvanic fluid produces, according to Humboldt, the 
lame effedl in the animal economy as the eleftric fluid in 
the mixture of azote and oxygen. It is this galvanic fluid 
that, being conveyed by the nerves, brings about the com¬ 
binations of the oxygen with the different acidifiable bafes 
of the fibres ; but, when the nerve of a part is tied,it pre¬ 
vents the fluid from paffing, which explains the reafon of 
the irritability being deftroyed. 
The oxygen neceffary for thefe unions is carried by 
the arterial blood in the courfe of circulation; and the 
acidifiable bafes, which are to unite with it, are found to 
Vol. XX. No. 1372. 
337 
be already prefent in the fibre. He found that every 
tiling that augments too much the quantity of the acidi¬ 
fiable bafes diminifhes the irritability ; and that every 
thing that increafes too much the quantity of oxygen 
likewife diminifhes it; and he thinks it very probable, 
that the fame takes place with refpedl to the proportion 
of the galvanic fluid. It is therefore only in a juft equi¬ 
librium of thefe principles that the neceffary irritability 
of the parts ccnfifts. 
Upon thefe principles this philofopher thus explains 
the produdtion of mufcular motion : “ In a ftate of re- 
pofe, the nerve being inferted in the mufcles, the galva¬ 
nic fluid is put into equilibrium in organs that touch each 
other. "The fpontaneous motion is made by a furcharge 
of galvanic fluid into the nerve. It appears that the in- 
ftant we wifli to make a motion, the galvanic fluid pro¬ 
duced in the brain is carried en maffe towards the part 
that ought to move, and furcharges the nervous fibres. 
A difcharge from the nerve is then made into the mufcles. 
The particles of thefe laft, animated by increafed affini¬ 
ties, approach each other; and it is this that conftitutes 
the phenomena of mufcular motion. 
Cullen, as we have feen, fell into the error of confound¬ 
ing irritability and fenfibility. Darwin did the fame. 
Haller fixed the feat of fenfation in the nerves, and con¬ 
ceived that it was produced by the circulation of a fluid 
through their fibres. The fame opinion was more ftre- 
nuouily fupported by Cullen. It is thus illuftrated by a 
difciple of the latter. “ It is probable that in each nervous 
fibril an elaftic fluid is inherent, forming, from the 
moment of animation, apart of it; differing, however, 
according to the ftate of the conftitution, in power, mo¬ 
bility, and perhaps in other qualities. Of this fluid the 
nerves are condublors, and are furrounded in their courfe 
by nonconducting membranes, while the fame membrane 
lines every part of the brain, and is carried into the 
deepeft cavities, guarding with particular attention the 
flighted aperture. In this view fanguiferous veffels are 
chiefly ufeful in nourifbing this medullary fubltance ; and 
they appear to be neceffary alfo in adapting the nerves to 
their office ; for, when the circulation is greatly increafed, 
the fenfibility is mere acute; and when it languifnes, or 
is deftroyed, the nervous energy foon fliares the fame 
fate. This fluid mull neceffarily be an elaftic one ; and 
impreffions are apparently conveyed through it by vibra¬ 
tions. It does not follow’ from hence, that the nerves vi¬ 
brate like mufcular cords; or that, in every the flighted 
motion, apportion is conveyed from the brain. The 
elafticity of the fluid is proved from the momentary con¬ 
tinuance of the impreffion after the caufe is removed ; 
and vibration is a term employed in many branches of 
philofophy as a means of communicating motion, without 
any diftindt application. If we touch an objedt with a 
flick, or with a metallic rod, we perceive through it the 
impreffion, and, in a general way, the nature of the fub- 
ftance. The impreffion mull be conveyed by fomething; 
and, whatever that fomething is, it may as well convey 
impreffions through the nerves as through the rod. But 
through the nerves only can itaffedl the brain, and pro¬ 
duce an idea, or fome change in the brain or its fluid, con- 
nedled with the nature of the objedl, and which conveys 
to the mind fome peculiar and diferiminated impreffion 
which it afterwards retains.” New London Medical Dic¬ 
tionary, vol. i. p. 398. 
The peculiar opinions of Darwin on the fubjedl of fen¬ 
fation merit notice, though on examination they will be 
found to be fcarceiy fo original (when corredl) as 
the curious jargon they are clothed in would at firft lead 
us to fuppofe. The immediate organs of fenfe are, by 
this author, afferted to coniift, like the mufcles, of moving 
fibres. The contradlions of the mufcles and of the 
organs of fenfe are comprehended under what are called 
Jibrous motions, in contradiftindlion to the JenJ'orial motions, 
or the changes which occafionally take place in the fetifo- 
rium. By this latter term is underftood, not only the 
4 R medulla 
