By this is meant our power of judging the exact position of any point or points of contact which may be applied to the skin. Thus, if the point of a pin be gently laid on a sensitive part of the skin we know at once when we are touched, and if a second pin be applied in the same neighborhood, we feel the two points of contact and can judge of their relative position. When we feel anything, we receive impulses from many points of contact bearing varied relationships to each other, and thus become conscious of a rough or smooth surface.

The delicacy of the sense of locality differs very much in different parts of the skin. It is most accurate in those parts which have been used as touch organs during the slow evolution of the animal kingdom.

The method of testing the delicacy of the sense of locality is that of applying the two points of a compass to different parts of the skin, and by varying their position, experimentally determine the nearest distance at which the two points give rise to distinct sensations. The following precautions must be attended to in carrying out this experiment: 1. The points must be simultaneously applied, or two distinct sensations will be produced at abnormally small distances. 2. The force with which the points are applied must be equal' and minimal, because excessive pressure causes a diffusion of the stimulus and a blurring of the tactile sense. 3. Commencing with greater and gradually reducing the distance of the points enables a person to appreciate a less separation than if the smaller distances were used at first. 4. The duration of the stimulus; two points of contact being distinguished at a much nearer distance if the points be allowed to rest on the part, than when they are only applied for a moment. 5. The temperature and material of the points should be the same. 6. Moisture of the surface makes it more sensitive. 7. Previous or neighboring stimulation takes from the accuracy of the sensations produced. 8. The temperature of the different parts of the skin should be equal, as cold impairs its sensibility.

The following table gives approximately the nearest distances at which some parts, which may be taken as examples of the more or less sensitive regions of the skin, can recognize the points of contact by their giving rise to two distinct sensations: -

Tip of the tongue

... 1 mm

Palmar aspect of the middle finger tip,.....

2 "

Tip of the nose

4 "

Back of the hand

15 "

Planter Surface of great toe

. 18 "

Fore arm, anterior surface, ..........

. 40 "

Front of thigh

55 "

Over ensiform Cartilage

. 60 "

Between scapulae

. 70 "

If one point of the compass be applied to the same spot, and the other moved round so as to mark out in different directions the limits at which the points can be distinguished as separate, we get an area of a somewhat circular form, for which the name sensory circle has been proposed. It would be convenient to explain this on the simple anatomical basis that the impressions of this area were carried by one nerve fibre to the brain, and thus but one sensation could be produced in the sensorium. We know this cannot be the true explanation, from the following facts: 1. No such anatomical relationship is known to exist. 2. By practice we can reduce the area of our sensory circles in a manner that could not be explained by the development of new nerve fibres. 3. If the two points of the compass be placed near the edges of two well-determined neighboring sensory circles, and so in relation with the terminals of two nerve fibres, they will not give distinct impressions; they require to be separated as much as if they were applied within the boundary of one of the circles where they also give rise to the double perception.

To explain better the sense of locality, it has been supposed that sensory circles are made up of numerous small areas, forming a fine mosaic of touch fields, each of which is supplied by one nerve fibre, and that a certain number of these little fields must intervene between the stimulating points of the compass in order that the sensorium be able to recognize the two impulses as distinct. For, although every touch field is supplied by a separate nerve fibril which carries its impulses to the brain, and is therefore quite sensitive, the arrangement of the cells in the sensorium is such that the stimuli carried from two adjoining touch fields are confused into one sensation. Thus, when an edge is placed on our skin, we do not feel a series of points corresponding to the individual fields with which it comes in contact, but the confusion of the stimuli gives rise to an uninterrupted sensation, and we have a right perception of the object touched.