732 PHYSIOLOGY. 



sensation of pressure. When the pressure is sufficiently increased 

 there results pain. This in turn disappears when the pressure is 

 increased to disorganization of the tissues. 



Pressure varies not only in intensity, but in extent. No mat- 

 ter how the latter may be limited, the pressure always affects at 

 least more than one peripheral nerve-ending. 



When tactile sensations are very light and succeed one another 

 rapidly, a large number of nerves is stimulated. The sensation ex- 

 cited is a peculiar one : that of tickling. 



Traction upon the hair and nails determines pain much more 

 rapidly than does pressure. 



LIQUIDS. Liquids applied .at the temperature of the skin exer- 

 cise a uniform pressure upon all parts of the cutaneous surface 

 excepting those at the level of the surface of the fluid. 



If a finger be plunged into a heavy fluid, as metallic mercury, 

 the part submerged bears a pressure which decreases from below 

 upward uniformly. It is only at the surface of the liquid that a 

 marked inequality of pressure exists. It follows a circular line 

 which surrounds the finger at this level and can be plainly felt by 

 the individual. If a lighter fluid, as water, be used, the pressure 

 sensation is but very slight. 



COMPOUND TACTILE SENSATIONS. These may be simultaneous 

 or successive. Simultaneous tactile sensation may be either double 

 or multiple. Double sensations, whether of contact, pressure, or 

 traction, are shown only when the stimuli are applied at a certain 

 distance from one another. If the stimuli be near enough, the sen- 

 sation remains single even though the stimulus has been applied to 

 the skin in two places. The earliest systematic experiments upon 

 this subject were by Weber. He touched the various points of the 

 skin's surface with a pair of carpenter's compasses and then observed 

 the distance of separation necessary to give a distinct impression of 

 two points of contact. The instrument now used for this purpose is 

 the CBsthesi&meter. From the table compiled by Weber it is found 

 that the tip of the tongue is most sensitive, while the thigh and arm 

 are least so. In the case of the tongue, the threshold stimulus, the 

 minimum separation necessary for the impression of double contact 

 is but 1.1 millimeter; 67.6 millimeters are necessary in the case of 

 the thigh and arm. The connection between the mental and physical 

 conditions explains certain illusions of tactile sensations. Of these, 

 the best known is the so-called experiment of Aristotle. When a 

 pea or small ball is rolled between the crossed index and middle 



