(Ab irritare). Irritation is a term to which different meanings have been affixed, and this has occasioned some confusion among pathologists. The most obvious idea of irritation is the action of a mechanical or a chemical acrid, as a thorn under the skin, or the effluvium of ammonia in the nose. Similar irritation is the effect of poison in the habit, as of cantha-rides; of altered secretion, as in gonorrhoea; of unnatural contents in the stomach, as in heartburn. Irritation, however, is discovered by its effects, when not obvious to the sense, as in cases of scirrhi, worms, ossifications, or extravasated blood; and, independent of these, a peculiar state of the excitement of the nervous power occasions the most common impressions to become the source of pain and uneasiness. This state of excitement is sometimes owing to inflammation, which acts in a manner we shall afterwards explain (see Nervus and Tonus); sometimes to latent sources of irritation in the brain, sometimes perhaps to a change in the slate of the nervous power itself. We can no otherwise explain the effects of an east wind on some hypochondriacs, or a particular state of electricity of the air on persons peculiarly susceptible of its effects.

We have already had occasion to remark, that privations sometimes occasion what are called symptoms of irritation.'thus hunger produces restlessness and anxiety; the want of the degree of tension, either from external pressure or internal fulness, from the sudden emptying of any cavity, will occasion uneasiness, which has been styled a symptom of irritation. Internal feelings of this kind are sometimes opposed to pain; and the peculiar sinking in atonic inflammations, has been styled also a symptom of irritation, not, perhaps, with perfect propriety,but with sufficient distinctness when explained.

The effects of irritation are generally increased action; and, in the animal system, privations are, by an unaccountable solecism, considered as positive causes of increased action. This loose, illogical language arises from a want of distinction between increased and irregular action; for in spasms produced by causes destructive of life, in convulsions closing the last scene of mortal existence, it would be absurd to say that action, which always implies energy, is increased. We have already shown that in all these instances the power is diminished, and the action, in consequence, irregular. See Convulsions and Inirritantia.

Is, (Irritatio 4576 a fibre,) its plural is Hippocrates, and other writers, have used this term for both a fibre and a nerve.