TACTILE IMPRESSIONS. 541 



Tip of the tongue, . 1 mm. 



Palmar aspect of the middle finger-tip, . . . 2 " 



Tip of the nose, 4 " 



Back of the hand, 15 " 



Plantar surface of great toe, 18 " 



Forearm, anterior surface, 40 " 



Front of thigh, 55 " 



Over ensiform cartilage, 50 " 



Between scapulae, 70 " 



If one point of the compass be applied to the same spot, and 

 the other moved around so as to mark out in different directions 

 the limit at which the points can be distinguished as separate, we 

 get an area of a somewhat circular form, for which the name sen- 

 sory circle has been proposed. It would be very 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 the one sensation could be produced in the sensorium. But 

 we know this cannot be the true explanation, because of the fol- 

 lowing 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 develop- 

 ment 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 ; in fact they require to 

 be separated just as far as if they were applied within the boundary 

 of one of the circles where they also give rise to the double per- 

 ception. 



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 compasses 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 sensi- 

 tive, the arrangements in the sensorium are such that the stimuli 

 carried from two adjoining touch-fields are confused into one seusa- 



