THE SENSE ORGANS 169 



only the sensation of pressure at the surface of the mercury will remain 

 behind; in fact, eventually even the latter will appear only when the 

 finger is moved. 



8. Touch Sensations Modified by Movement. Touch the skin of the 

 dorsal surface of the arm with the tip of your index-finger. Note the 

 quality of the sensation. Draw the finger slowly across this surface. 

 The sensation of touch is changed into one of stroking, due in all prob- 

 ability to the activation of the nerve plexuses investing the roots of the 

 hairs. 



Feel any surface with the tip of your index-finger. Draw the finger 

 across the surface. Note that the sensation of simple contact is now 

 amplified by sensations of motion and space. 



9. Projection of the Sensations of Touch. Hold a metal rod be- 

 tween your fingers and draw its end across a roughened surface. The 

 sensation will be referred to the end of the rod, i. e., be projected beyond 

 the skin. 



Dip the elbow in cold water. The initial sensation of cold at the 

 point of contact with the water will soon give way to a similar sen- 

 sation in the region innervated by the ulnar nerve. 



10. Mechanical Stimulation of the Hot and Cold Spots. Close your 

 eyes and instruct the assistant to touch the dorsal aspect of your hand 

 with the blunt point of a pencil. In certain areas you will obtain a 

 sensation of cold, and in others, of heat. These spots are separated by 

 areas in which no sensations of temperature are evoked. 



Allow the assistant to draw the point of a pencil slowly across the 

 surface, noting that distinct points of cold flash out. Sensations of 

 heat are not so easily elicited. 



11. Thermal Stimulation of the Hot and Cold Spots. Employ a 

 metal rod, about 10 cm. in length and 1 cm. in diameter. Its pointed 

 end projects from a covering made of rubber tubing. Cool the rod in 

 ice-water and map out a circumscribed area upon the dorsal aspect of 

 the subject's hand. Mark the cold spots with black ink. Warm the 

 rod to 70 C. and proceed as before, marking the warm spots in red ink. 

 Observe that the latter are less easily found and that their stimulation 

 is followed by a longer latent period. 



12. Chemical and Electric Stimulation of the Temperature Spots. 

 Apply objects of different heat-absorbing power to the skin, such as 

 wool and a piece of metal. The latter feels colder because it gives rise 

 to a greater loss of heat, thereby stimulating the cold spots. 



Rub menthol upon the skin of the hand or forehead. A sensation 

 of cold is obtained because this agent renders the skin hyperesthetic. 



Identify a cold spot upon the dorsal surface of the hand. Employ 

 two electrodes, one pointed and the other flat. The former is applied 

 to the cold spot and the latter elsewhere upon the hand. Stimulate 

 with a weak induction current until a distinct sensation of cold is ob- 

 tained. 



13. "After-images" of Temperature. Place a cold coin on the 



