ALTERATIONS OF SENSIBILITY - Sensation may be morbidly keen, or it may be obtuse and even entirely lost, or it may be perverted ; each of these conditions extends, more or less, to the whole nervous system, or is limited to particular organs. With reference to all alterations of sensi bility, a distinction must be made between pain and tenderness : the one denotes the existence of some unusual stimulus, the other indicates increased susceptibility to any impression ; they are often present together in various conditions of disease (e. g., local inflammations), and we are apt to consider them as only different expressions of the same nervous phenomena. When they are taken as symptoms of cerebral disease, and when no local cause exists in the part in which the phenomenon is present, it is still more important to remember the exact idea which each conveys : the one is to be regarded as perverted sensation ; the other as morbid sensibility.
§ 1. General Alterations of Sensibility.—General tenderness of surface is not a symptom of much consequence when standing alone ; it is then commonly the result of hysteria, or mere nerv ous excitability ; if associated with causeless anxiety, depression, or dread, or with irascibility of temper or great elevation of spirits, it points to insanity. • Sensibility generally diminished is probably never seen except as the result of mental alienation, or as combined with general paralysis; but it must be remembered that it is not by any means a necessary concomitant of paralysis.
Perverted sensations affecting the whole system are similarly best seen in cases of mental delusion. Analogous phenomena are observed in the sensation of heat complained of by patients in Asiatic cholera, while the whole body is sensibly cold ; in the sensation of chilliness in fever, when the skin is morbidly hot to the touch ; and in the extreme cold and shivering of ague, or of severe rigor. The tingling and formication of jaundice, and similar sensations produced by the action of certain substances in peculiar idiosyncrasies, are scarcely to be regarded in the same light. The only one which really bears on our present subject is
that general sensation of pain and malaise which cannot be local ized by the patient, and is not to be accounted for by the condi tion of the blood, as in fever; this symptom is not to be lightly disregarded, and is often the precursor of more serious lesions of the nervous system.
§ 2. Alterations in the Sense of Sight.—Of local conditions, none deserve more consideration than those presented by the organs of vision, where the pupil so readily exhibits the increased or dimin ished sensibility of the retina, independent of the patient's voli tion. They consist of— a. Difference of size of the pupils on either side, which may with certainty be regarded ais evidence of severe lesion of one half of the brain ; it usually results from partial or complete in sensibility of one retina, and very rarely from increased suscep tibility or irritability : in the majority of instances it is a dilatation of one •pupil, and not a contraction of the other.
b. Morbilronalaction of both pupils associated either with (1) intolerance of light, pointing to inflammatory action; or (2) with insensibility more or less marked„especiall,y seen in coma and narcotism; or (3) with increased irritability, the pupils dilating pretty freely when light is withdrawn, but contracting unduly on its admission.
c. Morbid dilatation of both pupils : (1) with insensibility com plete, indicating pressure equally affecting both hemispheres, and hence most commonly seen in effusion of fluid in the ventricles ; (2) with oscillating movements when light is withdrawn, and again suddenly admitted—a condition most commonly found in the transition stage from inflammation to exudation in the hydro cephalic forms of disease ; (3) with sluggish movements, which only show an obtuseness in the perception of light, and the ex citement of reflex action, the pupil dilating largely, and contract ing feebly when light is withdrawn and again admitted—a com mon condition in fever ; (4) a similar state of the pupil is also produced by belladonna, hyosoyamus, &c.