102 HUMAN PHYSIOLOGY 



because the first has high thermal conductivity, the second 

 has not, and thus the former abstracts heat more rapidly 

 than the latter. 



Certain parts of the skin are stimulated by the withdrawal 

 of heat to give rise to sensations of cold, while others are 

 stimulated by the addition of heat. This may be demon- 

 strated by taking the cold point of a pencil and passing it 

 over the back of the hand when it will be felt as cold only at 

 certain points ; such points have been called cold spots, while 

 similar spots stimulated by the addition of heat are called 

 hot spots. (Experiment.) 



The acuteness of the temperature sense may be tested by 

 finding the smallest difference of temperature which can be 

 distinguished. This may be done by taking two test tubes 

 in which thermometers have been placed, and filling them 

 with water at slightly different temperatures, and then apply- 

 ing them to different parts of the skin. If the temperature 

 is very low or very high, differences of temperature are not 

 readily distinguished, and in fact painful sensations may 

 take the place of temperature sensations. But between 15 

 and 50 C. the power of distinguishing differences of tempera- 

 ture is fairly constant, but it varies in different parts of the 

 body. Over the cheek as small a difference as '2 C. can be 

 appreciated, while on the back the difference must be as great 

 as -9 C. (Experiment.) 



The temperature sense is independent of the tactile sense. 

 The one may be lost and the other retained. It is probable 

 that the nerve endings in the deeper layers of epithelium 

 are connected with the temperature sense. But although 

 independent, the tactile and thermal senses influence one 

 another. A cold body placed on the skin feels heavier than 

 a warm body, as may be shown by placing first a cold penny 

 and then a warm penny on the skin of the forehead. 



III. Vision. 



While the addition to and withdrawal from the surface of 

 the body of the slower waves of ether which are the basis of 

 heat act upon the special nerve terminations in the skin to 

 give rise to sensations of heat and cold, a certain range of 



