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action, irritable, time, stimuli, cold, quantity, body, mus, stomach and sensible

IRRITABILITY, in physiology, is the property peculiar to the muscles, by which they contract upon the applica tion of certain stimuli, without a consci ousness of action. Haller and other phy siologists denominate that part of the hu man body irritable, which becomes short er by being touched : very irritable, if it contracts upon a slight touch. They call that a sensible part of the human body, which, upon being touched, transmits the impression of it to the mind : on the con trary, they call that insensible, which, be ing burnt, torn, cut, &c. occasions no sign of pain or convulsion, nor any sort of change in the situation of the body. It is inferred that the epidermis is insensible ; that the true skin is the most sensible part of the body ; that the fat and cellular membranes are insensible ; and the mus cular flesh sensible, the sensibility of which he ascribes rather to the nerves than the flesh itself. The tendons, hav ing no nerves distributed among them, are deemed insensible. Irritability then is the distinguishing characteristic be tween the muscular and cellular fi bres. Irritability differs from sensibility, and is not proportioned to it: the intestines are less sensible than the stomach, but more irritable ; the heart is very irritable, though it has but a small degree of sen sation. The laws of irritability, accord ing to Dr. Crichton, are : 1. After every action in an irritable part, a state of rests or cessation from motion, must take place, before the irritable part can be again in cited 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 be comes relaxed, notwithstanding all our endeavours to the contrary, and remains a certain time in that relaxed state, before it can be again thrown into action. 2. Each irritable part has a certain portion or quantity of the principle of irritability which is natural to it, part of which it loses during action, or from the applica tion of stimuli. 3. By a process, wholly unknown tons, it regains this lost quantity during its repose or state of rest. In or der 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 stimuli which are calculated to act on that part are withdrawn, or withheld for a certain length of time, because then no action can take place ; while, on the other hand, the application of stimuli causes it to be exhausted, or to be deficient, not only by exciting action, but by some secret in fluence, the nature 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 de prived of its irritability by powerful sti muli, and yet no apparent cause of mus cular or vascular action takes place at the time. Thus a certain quantity of spirits taken at once into the stomach kills al most as instantaneously as lightning does : the same thing may be observed of some 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 arteries, if by any accident it gets into the stomach, 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 di versity which we observe in the action of various irritable parts : thus the mos clesof voluntary motion can remain a long time in a state of action, and if it be con tinued as long as possible, another consi derable 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 mus Iles of the intestines have also a quick ae. taon and short rest. 6. All stimuli pro duce 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 increased, and it glows with heat ; if the hand be approach ed still nearer, the action is increased to such an unusual degree as to occasion redness and pain ; and if it be continued too long, real inflammation takes place ; but if this heat be continued, the part at least loses its irritability, ands sphacelus or gangrene ensues. 7. The action of every stimulus is in an inverse ratio to the frequency of its application. A small quantity of spirits taken into the stomach, increases the action of its muscular coat, and also of its various vessels, so that di gestion is thereby facilitated. If the same quantity, however, be taken frequently, it loses its effect. In order to produce the same effect as at first, a larger quan tity is necessary ; and hence the origin of dram-drinking. 8. The more the irri tability of a part is accumulated, the more that part is disposed to be acted upon. It is on this account that the ac tivity of all animals, while in perfect health, is much livelier in the morning than at any other time of the day ; for during the night the irritability of the whole frame, and especially that of the muscles destined for labour, viz. the mus cles of voluntary 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 accrues to a famished person upon first taking in food. 9. If the stimuli which keep up the action of any irritable body be withdrawn for too great a length of time, that process on which the forma tion of the principle depends is gradually diminished, and at last entirely destroy ed. When the irritability of the system is too quickly exhausted by heat, as is the case in certain warm climates, the appli cation of cold invigorates the frame, be cause cold is a mere diminution of the overplus of that stimulus which was caus ing the rapid consumption of the princi ple. Under such, or similar circumstances, therefore, cold is a tonic remedy ; but if in a climate naturally cold, a person were to go into a cold bath, and not soon re turn into a warmer atmosphere, it would destroy life just in the same manner as many poor people, who have no comforta ble dwellings, are often destroyed from being too long exposed to the cold in winter. Upon the first application of cold the irritability is accumulated, and the vascular system therefore is disposed to great action ; but after a certain time all action is so much diminished, that the process, whatever it be, on which the formation of the irritable principle de pends, is entirely lost. See Dr. Crichton on Mental Derangement for more on this subject