IRRITABILITY. m 
Irritability, be fays, is fo different from fenfibility, that 
the molt irritable parts are not at all fenfible, and vice verfa. 
He alleges fa£ls to prove this pofition, and alfo to demon- 
ifrate, that irritability does not depend upon the nerves, 
which are not irritable, but upon the original formation 
of the parts which are fufceptible of it. Irritability, he 
fays, is not proportioned to fenfibility ; in proof of which, 
he obferves, that the inteftines, though rather lefs fen¬ 
fible than the flomach, are more irritable ; and that the 
heart is very irritable, though it has but a finall degree of 
fenfation. 
Irritability, according to Dr. Haller, is the diftinguifh- 
ing charadteriftic between the mufcular and cellular fibres; 
•whence he determines the ligaments, periodeum, meninges 
of the brain, and all the membranes compofed of the cel- ■ 
•lular fubilance, to be void of irritability. The tendons 
are unirritable; and, though he does not ablolutely deny 
irritability to the arteries, yet his experiments on the aorta 
produced no contraction. The veins and excretory dudts 
are in a fmall degree irritable, and the gall-bladder, the 
ductus choledocluis, the ureters and urethra, are only af- 
fedied by a Very acrid corrofive ; but the ladteal veffels 
are confiderably irritable. The glands and mucous finufes, 
the uterus in quadrupeds, the human matrix, and the ge¬ 
nitals, are all irritable; as are alfo the mufcles, particu¬ 
larly the diaphragm. The cefophagus, ftomach, and .in¬ 
teftines, are irritable: but of all the animal organs the 
heart is endued with the greatefl irritability. In general, 
there is nothing irritable in the animal body but the 
mufcular fibres: and the vital parts are the molt irritable. 
This power of motion, arifing from irritation, is fuppofed 
to be different from all other properties of bodies, and 
probably refides in the glutinous mucus of the mufcular 
fibres, altogether independent of the influence of the foul. 
The irritability of the mufcles is faid to be deftroyed by 
drying of the fibres, congealing of the fat, and more efpe- 
cially by the ufe of opium in living animals. The pby- 
iiological fyftern, of which an abftraft has been now given, 
has been adopted and confirmed by Caftell and Zimmer- 
mann, and alfo by Dr. Brocklefby, who fuggefts, that irri¬ 
tability, as diftinguifhed from fenfibility, may depend 
upon a feries of nerves different from fuch as ferve either 
for voluntary motion or fenfation. This dodlrine, how¬ 
ever has been controverted by M. le Cat, and particularly 
by Dr. Whytt, in his Phyfiological Effays. 
Irritability in Plants. The high degree of irrita¬ 
bility exhibited by the Mimofa fenfitiva, or fenfitive-plant, 
is well known: but, by the recent experiments of Dr. Car- 
radorri, an ingenious foreign naturalift, it appears, that 
many other vegetables, or parts of vegetables, at certain 
periods of their growth, exhibit the fame phenomena. 
The doftor’s firft experiments were made on the LaCtuca 
fativa, or garden-lettuce; and from thefe it would appear, 
that if a lettuce plant be gently touched with the finger, 
when it is in feed, and particularly when in flower, it 
may be obferved to emit, at the place fo touched, a milky' 
liquor, in the form of very minute drops. This pheno¬ 
menon occurs however only in the fmall amplexicaul 
leaves ot the branches, and the leaflets of the calyxes, or 
.flower-cups. The contact of any folid body, however 
fimootb, even the application of the flighted: ftimulus, 
operates to produce the emiffion of this liquor. A blade 
of grafs, or any other pointed body, applied in the molt 
gentle manner, excites in the part fuch a degree of irrita¬ 
tion as to make it throw out a milky humour in the form 
of jets, which an’attentive eye may readily perceive fpurt- 
ing into the air to fome didance. Neither a drop of wa¬ 
ter, nor of the nitrous, fulphureous, or muriatic, acids, 
when applied very gently, fo as to produce np mechanical 
impulfe, appeared to Dr. Carradorri capable of exciting 
the lead emiffion from the molt irritable" parts of the 
plant; but, when one or more drops of any fluid were 
made to fall upon it, they never failed to produce an emif- 
rflon of the milky juice. Neither the fmoke of tobacco, 
Tior the fumes of the nitric and fulphuric acids, produced 
the lead effedt. Dr. Carradorri next dircdied upon it a 
V£L, XI. No, 7 6z. 
dream of air, by blowing through a draw with all his 
force; but, unlefs when he approached extremely near, 
not the fmalled effedt was produced. The application of 
heat by means of a burning coal or red-hot iron, appeared 
to have no otherinfluen.ee but that of dedroyingthe por¬ 
tion of the plant near to which they were brought; neither 
did the action of cold, nor the application of ice, produce 
in this vegetable any fenfible change. But the dighteft 
touch of an infedt was fulficient to excite the irritability 
of the plant, and produce an emiffion of the milky liquor. 
It is extremely intereding to obferve, fays Dr. Carradorri, 
how the ants, which frequently creep upon this plant, in 
order to colledt, and carry off its feeds, are entangled in 
the milky juice exuding from it, in confequence of the 
flight impreflion made by their feet. 
Thefe plants, torn up by the roots, or their branches 
detached from the flem or trunk, furnifh, on the applica¬ 
tion of ftimuli, the fame exudation, in whatever place they 
may be kept, fo long as they pofiefs a certain degree of 
vitality, or vegetative power. Having torn up a lettuce 
when in full flower, and pofleffing the mod certain ligns 
of irritability, Dr. Carradorri iinmerfed it flowly, while 
holding it in an upright pofition, in a ved'el of water; as 
the water came into contadi with the leaves of the plant, 
or the leaflets of the calyx, it excited them to ejedt the 
milky liquor with which they were furnifhed. On applying 
dimuli to the plant when under water, a fimilar emidioa 
of juice took place, as when it was excited in the air; and, 
when the excitement was fomewhat greater, he could dif- 
cern the milky liquor thrown by jets into the water. This 
unquedionably mud have proceeded, he obferves, from a 
power inherent in the plant itfelf, for it cannot be fup- 
poied that the milky juice could be emitted through 
the organic pores of the lettuce, unlefs it had been pro¬ 
pelled by a conftridtive power, or fyltolic motion, in the 
vafcular fyftem ; and, as this power is excited by the Am¬ 
ple contadt, or application, of any body capable of pro¬ 
ducing excitement, and ceafes uniformly when the dimulut 
is withdrawn, it mud, on that account, be referred to a 
principle of irritability. 
This law, which is common to living matter, Dr. Carra- 
dorri conceives he has fully demondrated to prevail in the 
lettuce; fince, as often as any folid body, whether rough 
or fmooth, round or pointed, was brought into contact 
with this plant when in dower, it never failed to excite 
the part fo touched to emit or ejedt a milky fluid, in the 
fame way as mufcular contractions are excited in animal* 
by the application of dimuli; this effedt could not, how¬ 
ever, be prolonged to an indefinite period by the continued 
application of the exciting body, though it was renewed 
after a fliort interval ; in the fame manner as mufcular con¬ 
tractions cannot be extended beyond a certain period by 
the prefence of the irritating caufe, till after the lapfe of 
fome time, when the mufcular fibre, having recovered its 
excitability, again becomes fufceptible of excitement. 
Such an effect cannot, Dr. Carradorri goes on to ob¬ 
ferve, be referred to any mechanical caufe, fince he fre¬ 
quently found that the emiffion of moidure was not in 
proportion to the collifion, or the preffure of the body- 
applied to a vegetable, but to the excitement which it 
produced; hence it is evident, that the phenomenon in 
quedion is merely the effedt of excitement or dimulation. 
Befides, he never perceived that a greater excitement' was 
produced in the plant in proportion to its being drongly 
rubbed, or even pricked by a very fharp body ; on the 
contrary, it always appeared to be more confiderable when 
the plant was only .llightly rubbed by the finger, or gently 
touched by a blade of grafs. 
Heat, cold, drong odours, volatile fluids, &c. are all, 
it is well known, capable of abting on the irritability of 
fome vegetables, as for example, on the fenfitive-plant j 
but, according to Dr. Carradorri’s experiments, they do 
not operate with fufficient power to excite the irritability 
of rbe lettuce. Hence, it fhould feem, that the irritability 
of this plant is not fenfible to every ftimulus. 
After detailing thefe experiments, Dr. Canadorai makes 
5 Cl th« 
