THE SKIN AS AN ORGAN OF SENSATION 93 



will be found that a weight touching a hair may be felt, even though 

 its contact is not perceived when it is applied directly to the skin. 



39. Estimation of Weight by Sense of Pressure. Rest the back of the 

 hand upon some easy support, and place on the pal in a small wooden 

 or pasteboard disk. Upon the latter place different weights. Find 

 the least difference in weight that can be detected. A great variety 

 of weights can be obtained by loading empty cartridges with shot to 

 any desired extent. If the cartridges are all of the same size, then the 

 person experimented upon can not estimate the weight by sight. Sev- 

 eral pupils should be experimented upon, to show variation in acute- 

 ness of pressure sense. 



40. The Muscular Sense. Modify the preceding experiment by 

 having the weights lifted instead of simply allowing them to press on 

 the hand. It will be found that smaller differences can be detected 

 than by pressure alone. Demonstrate that a weight lifted slowly 

 seems heavier than one lifted rapidly. 



41. Sensations of Heat and Cold. That there are in the skin two 

 distinct varieties of nerve endings of the temperature sense can be 

 very easily demonstrated by carefully stimulating any certain area of 

 the skin with hot and cold bodies. Let a square be marked off with 

 ink, on the forearm, and a pointed brass rod be heated and then care- 

 fully drawn across this square in parallel lines in various directions. 

 Here and there a sensation of heat will appear distinct from the sensa- 

 tion of contact. The hot spots should be marked with ink dots, as 

 they are recognized. Then in a similar way go over the square with 

 a cold brass rod. Cold spots will occasionally appear, in almost every 

 case distinct from the hot spots. The cold spots should be marked in 

 ink of a different color from that of the hot spots. 



