EXERCISE 89 



Sensitiveness to heat or cold, on the surface of the body, depends upon the presence of specific 

 nerve endings, which are distributed rather unevenly in the skin. 



Problem, (i) What is the distribution of the hot points and of the cold points in different parts o 

 the skin ? (2) Is there any relation between the positions of hot points and cold points ? 



What to use. Some pieces of wire or wire nails, about two inches long ; some hot water (abou 

 80 C.) ; some cold water (from 5 to 10 C.) ; two students working together. 



What to do. Keep the nails in the water, some hot and some cold, except while in actual use 

 Touch the tip of a hot nail lightly to the skin of the palm of the hand (using the lines on the skin as 

 a guide), then about one eighth of an inch farther, and so on, noting at each contact whether there is 

 a distinct sensation of heat. Locate in this way, on the palm of the hand, from twenty to thirty ho 

 points. Do the same on the cheek or on one of the fingers. Repeat with the cold nails. Change to 

 a fresh nail as soon as the temperature of the one in use becomes too much like that of the sur 

 rounding air. 



Record. Make a diagram of the areas studied. Locate the distribution of the hot points by means 

 of red-ink dots, and that of cold points by means of black dots. Describe the results and answer the 

 two questions in the Problem. 



Questions. 1. About how close together are the hot points on the skin ? 



2. Which kind of nerve ending is more frequent, the cold-perceiving or the hot-perceiving ? Would 

 this be true in all parts of the body ? 



[96] 



