168 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



Cross the fingers in the same way and rub them against the tip of 

 the nose. Describe the sensation. 



Annotation. Ordinarily the tactile corpuscles upon the outer surface of the 

 index-finger and inner surface of the middle finger act in unison, producing har- 

 monious impressions. If the corpuscles upon the inner aspect of the index-finger are 

 now brought in relation with those upon the outer aspect of the middle finger, two 

 impulses must result in consequence of the stimulation of dissimilar tactile points. 



6. Peculiar Phenomena and Illusions of Touch and Pressure. 



Place the cross-section of a tube against the skin of the forearm. An 

 impression of a transversely oval object is produced thereby. 



Separate the points of a compass about 2 cm. and draw them in a 

 straight line downward across the skin of the forearm, wrist, palm of 

 hand, and fingers. A sensation of a single line opening up into two is 

 produced; moreover, these lines appear to be more widely apart at the 

 wrist, to converge in the palm, and to diverge toward the tips of the 

 fingers. 



Close your eyes, and follow the circumference of a round object 

 first with a short rod and then with a long rod, held between your fingers 

 in the usual way. The object will appear to be much larger when out- 

 lined with the short rod. 



Apply the points of a compass simultaneously to the skin of the 

 subject, his eyes being closed. Allow the subject to obtain a clear im- 

 pression of the distance between the points. Apply only one of the 

 points, and move this point rapidly an equal distance across the skin. 

 In the former instance the distance will seem greater. 



Make a knot in a coarse thread about 30 cm. in length. Allow the 

 subject to hold the knot between the thumb and index-finger of his 

 right hand, his eyes being closed. Pull the thread through his fingers 

 first slowly and then more quickly. Is the subject capable of estimating 

 the length of the thread? 



Apply a comb to the dorsal surface of the subject's hand, his eyes 

 being closed. Ask him to indicate the length of the comb actually 

 applied. It will appear shorter to him than it really is. Draw the 

 same distance of comb slowly across the surface. It will now seem longer 

 than it actually is, because movement leads to an exaggerated sensation 

 of length. 



Draw the head of a pin rapidly to and fro across the skin of the fore- 

 arm of the subject, his eyes being closed. The subject will perceive 

 the motion some time before he is able to determine its direction. 



Touch your forehead with your finger. The finger "feels" the fore- 

 head. Rapidly draw the finger across the skin of the forehead. The 

 sensation will now be referred to the forehead. 



7. Adaptation to Touch Sensations. Place an object, such as a 

 cork, upon the skin of the forearm. The initial sensation of pressure 

 will gradually give way to an indifferent sensation. 



Dip your index-finger into a tube filled with mercury. Presently 



