The skin is described commonly as an apparatus of touch, a sense which resides in a special manner in the terminal parts of the extremities and the lips. It is, however, distributed more or less over the whole surface, varying in acuteness in different parts of the body. Dr. Carpenter refers, in his Principles of Physiology, to the researches of Prof. E. H. Webber and Dr. Ballard in reference to the relative sensitiveness in different parts of the skin. One test was that of employing the legs of a pair of compasses, the points of which, guarded with pieces of cork, were brought together as close as possible, until the smallest distance at which they could be felt to be apart from one another was reached. This was termed by Dr. Graves as the limit of confusion. A table is given, showing the limit of confusion, from which the following instances are taken. In the first place, the point of the tongue proved to be most sensitive, as it was capable of appreciating the two points of the compass when the distance between them was only half a line. On the palmar surface of the third finger it was one line; on the red surface of the lips, two lines; on the tip of the nose, three lines; on the skin of the cheek, five lines; on the mucous membrane of the gums, nine lines; on the skin over the sternum or breast, twenty lines; on the skin over the spine of the back, thirty lines. Investigators who followed up Prof. Webber's experiments found, as might have been expected, a considerable amount of individual variation, some persons being able to appreciate the points at a half, even at a third, of the distance required by others.

The sensibility in the skin to impressions of weight has also been tested by different investigators, and it was found that, on the face, the pressure of a portion of elder pith presenting a surface of nearly -1/3- inch square, and weighing only 1/33 grain, could be distinguished, whilst the tips of the fingers required a weight of 1/3 grain and more, the toes as much as 8 grains, before any sensation of pressure was felt. In regard to temperature, it was found that the left hand was more sensitive than the right, although the right has the sense of touch most acutely developed. The curious fact was also noticed, that a weaker impression made on a large surface conveys the idea that it is more powerful than a stronger impression on a small surface; thus, if the forefinger of one hand be immersed in water at a temperature of 104°, and the whole of the other at 102°, the cooler water will be felt to be the warmer; further, it is a known fact that water in which a finger can be held will scald the whole hand that is plunged into it. From these experiments it may be gathered that the sense of touch does not afford much assistance in obtaining accurate information as to the temperature of bodies. Indeed, it is well known that the temperature of the surface of the person who makes the experiment has its effect. For instance, a cold hand will appreciate a very slight rise of temperature in anything it touches, whereas it would distinguish little or no change in touching a body which was nearly the same temperature as itself.