IRRITABILITY. 
are less sensible than the stomach, but more 
irritable : the heart is very irritable, though 
it has but a small degree of sensation. 
The laws of irritability, according to Dr. 
Crichton, are ; 1. After every action in an 
irritable part a state of rest, or cessation 
from motion must take place before the ir- 
ritable part can be again incited to action. 
If by an act of volition we throw any of our 
muscles into action, that action can only be 
continued for a certain' space of time; the 
muscle becomes relaxed, notwithstanding 
all our endeavours to the contrary, and re- 
mains a certain time in that relaxed state, 
before it can be again thrown into action, 
g. Each irritable part has a cbrtain portion 
or quantity of the principle of irritability 
w'hich is natural to it, part of which it loses 
during action, or from the application of 
stimuli. 3. By a process wholly unknown 
to, us it regains this lost quantity during its 
repose or state of rest. In order to express 
the different quantities of irritability in any 
part, we say that it is either more or less 
redundant, or more or less defective. It 
becomes redundant in a part when the sti- 
muli which are calculated to act on tliat 
part are withdrawn, or withheld for a certain 
length of time, because then no action can 
take place : while, on the other hand, the ap- 
plication of stimuli causes it to be exhausted, 
or to be deficient, not only by exciting ac- 
tion, but by some secret influence, the na- 
ture of which has not yet been detected; 
for it is a circumstance extremely deserving 
of attention, that an irritable part or body 
may be suddenly deprived of its irritability 
by powerful stimuli, and yet no apparent 
cause of muscular or vascular action takes 
place at the time. Thus a certain quantity 
of spirits taken at once into the stomach 
kills almost as instantaneously as lightning 
does ; the same thing may be observed of 
gome poisons, as opium, laurel-water, the 
juice of some poisonous vegetables, &c. 
4 . Each irritable part has stimuli which are 
peculiar to it ; and which are intended to 
support its natural action: thus blood,which 
is the stimulus proper to the heart and arte- 
ries, if by any accident it gets into the sto- 
mach, produces sickness or vomiting. 5. 
Each irritable part differs from the rest in 
regard to the quantity of irritability which 
it possesses. This law explains to us the 
reason of the great diversity which we ob- 
serve in the action of various irritable parts : 
tlius the muscles of voluntary motion can 
remain a long time in a state of action, and 
if it be continued as long as possible, ano- 
ther considerable portion of time is required 
before they regain the irritability they lost ; 
but the heart and arteries have a more 
short and sudden action, and their state of 
rest is equally so. The circular muscles of 
the intestines have also a quick action and 
short rest. 6. All stimuli produce action 
in proportion to their irritating powers. As 
a person approaches his hand to the fire, the 
action of all the vessels in the skin is in- 
creased, and it glows with heat ; if the hand 
be approached still nearer, the action is in- 
creased to such an unusual degree as to oc- 
casion redness and pain ; and if it be conti- 
nued too long, real inflammation takes 
place ; but if this heat be continued, the 
part at last loses it irritability, and a sphace- 
lus or gangrene ensues. 7. The action of 
every stimulus is in an inverse ratio to the 
frequency of its application. A small quan- 
tity of spirits taken into the stomach in- 
creases the action of its muscular coat, and 
also of its various vessels, so that digestion 
is thereby facilitated. If the same quantity, 
however, be taken frequently, it loses its 
effect. In. order to produce the same ef- 
fect as at first, a larger quantity is necessary ; 
and hence the origin of dram-driaking. 
8. The more the irritability of a part is 
accumulated, the more that part is disposed 
to be acted upon. It is on this account 
that the activity of all animals, while in per- 
fect health, is much livelier in the morning 
than at any other time of the day; for dur- 
ing the night the irritability of the whole 
frame, and especially that of the muscles 
destined for labour, viz. the muscles of vo- 
luntary action, is re-accumulated. The 
same law explains why digestion goes on 
more rapidly the first hour after food is 
swallowed than at any other time ; and it 
also accounts for the great danger that ac- 
crues to a famished person upon first taking 
in food. 9. If the stimuli which keep up 
the action of any irritable body be with- 
drawn for too great a length of time, that 
process on which the formation of the prin- 
ciple depends is gradually diminished, and 
at last entirely destroyed. When the irri- 
tability of the system is too quickly ex- 
hausted by heat, as is the case in certain 
warm climates, the application of cold invi- 
gorates the frame, because cold is a mere 
diminution of the overplus of that stimulus 
which was causing the rapid consumption of 
the principle. Under such, or similar cir- 
cumstances, therefore, cold is a tonic re- 
medy ; but if in a climate naturally cold, a 
person were to go into a cold bath, and not 
