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 THE PERCEPTION OF SPACE. (l.) 19 that " they change, when we pass from one point of the skin to its neigh- bour, with very different degrees of rapidity. On delicately feeling parts, used principally for touching, such as the finger-tips, the difference of sensation between two closely approximate points is already strongly pronounced ; whilst in parts of lesser delicacy, as the arm, the back, the legs, the disparities of sensation are observable'only between distant spots." The internal organs, too, have their specific qualia of sen- sation. An inflammation of the kidney is different from one of the liver ; pains in joints and muscular insertions are distinguished. Pain in the dental nerves is wholly unlike the pain of a burn. But very important and curious simi- larities prevail throughout these differences. Internal pains, whose seat we cannot see, and have no means of knowing unless the character of the pain itself reveal it, are felt by us where they belong. Diseases of the stomach, kidney, liver, rectum, prostate, &c., of the bones, of the brain and its membranes, are referred to their proper position. Nerve pains describe the length of the nerve. Such localisations as those of vertical, frontal or occipital headache of intra- cranial origin, force us to conclude that parts which are neighbours, whether inner or outer, may possess by mere virtue of that fact a common peculiarity of feeling, a respect in which their sensations agree, and which serves as a token of their proximity. These local colourings are, moreover, so strong that we cognise them as the same, throughout all contrasts of sensible quality in the accompanying perception. Cold and heat are wide as the poles asunder ; yet if both fall on the cheek, there mixes with them something that makes them in that respect identical, just as, contrariwise, despite the identity of cold with itself wherever found, when we get it first on the palm and then on the cheek, some difference comes, which keeps the two experiences for ever asunder. 1 1 Of the anatomical and physiological conditions of these facts we know as yet but little, and that little need not here be discussed. Some differ- ences there must be, either in the composition of the nerve-tissue or in the manner in which, in different places, it is affected by the tissues in contact with it when they themselves are touched. These latter mechanical con- ditions cannot however obtain in the case of the retina, the different points of which exhibit nevertheless a wonderfully delicately graded system of sensations dependent on locality alone. Two principal hypotheses have been invoked in the case of the retina. Wundt (Mensclun- u. Thierseelc, i. 214) called attention to the changes of colour-sensibility which the retina displays as the image of the coloured object passes from the fovea to the periphery. The colour alters and becomes darker, and the change, is more rapid in certain directions than in others. This alteration in general, how- ever, is one of which, as such, we are wholly unconscious. We see the sky as bright blue all over, the modifications of the blue sensation being inter-