SENS IBILITY.— (Fr. Sensibilite ; Germ. Empfindlichkeit). — This term, like Sensa tion, should be limited to signify the power which any organ or tissue of the body has, of causing changes inherent or excited in it to be perceived and recognised by the mind. The greater this power is in any tissue or organ, the more sensitive it is, — the greater the sensibility of the organ or tissue ; the less this power is, the less the sensibility of the organ, &c.
Sensibility, like Sensation, involves the power of affecting the mind through the body ; but as the mind, of its own mere motion, may excite the centre of sensation, so, by directing the attention specially to some particular tissue or organ, it may create a sensation which, will be referred to that part,and NNhich, by frequent repetition, may assume the nature of pain. No doubt many instances of hys terical pain are greatly aggravated by the mind being constantly directed to, and dwell ing upon, the painful part.
The term Sensibility is sometimes con founded with Irritability, especially by Psycho 'logical writers. IIaller has, with great pre cision, laid down the distinction between these two properties of tissues in the following words :— " Irritabilem partem corporis humani dico, qux ab externo align° contactu brevior fit ; valde irritabilem, qum a levi contactu, parum qum a valente demum causa in brevitatem cietur. Sentientem parteiu corporis humani appello cujus contractus animm representatur ; et in animalibus brntis, de quorum anitna non perinde liquet, eas pastes sentientes dico, quibus irritatis animal doloris et incommodi signs ostendit ; insensilem contra partem gum usta, scissa, puncta, ad destructionem usque cmsa, nullum doloris signum, convulsionem nullam, nullam in totius corporis situ muta tionem ex citat."* The sensibility of any part must bejudged of by the readiness with which changes in it are perceived by the mind. In general, highly
sentient parts, when stimulated, are capable of exciting movements in the muscles of neigh bouring parts ; thus, stimulation of the sole of the foot excites motions in the whole lower extremity ; the stimulation of any other part of the leg, whilst it might excite movements, would not produce them to the same extent. The difference is due to the greater sensi bility of the sole of the foot than of any other part of the integument of the lower extremity, and also to the peculiar connection of its sen tient nerves with the papillary texture of the skin.
The anatomical condition necessary for the developement of the greater or less sensibility in an organ or tissue, is the distribution in it of a greater or less number of sensitive nerves. Thus the anatomist can determine the degree to which this property is enjoyed by any tissue or organ by the aniount of nervous supply which his research discloses ; and physiological experiments and surgical opera tions furnish us with abundant evidence in confirmation of the, as it were, a priori sug gestions of the anatomist.
The sensibility of tissues is modified by disturbances of their nutrition, and thence in flammatory affections tend to increased sensi bility, and will even make parts sensitive which before were but slightly so. Thus the periosteum which in health is but slightly sensitive, becomes, tinder the influence of in flammation, exquisitely sensible.
It is necessary to add that the word sensi bility is also used, as applied to nerves, to signify their power of evolving the nervous force. Excitability is a better word for this purpose, and ought to be generally used, to ensure a greater exactness in the applica tion of physiological terms than has hitherto prevailed.
(R. B. Todd.)