NERVE 103 



2nd. The plexuses round the roots of hairs, and possibly in 

 the epidermis, which are stimulated when a small piece of soft 

 cotton- wool is drawn over the skin. This mioht be called 

 the kinetic stimulation of the tactile mechanism, and the 

 other the static stimulation. 



3. Reactions to Changes of Temperature. 

 Thermal Sense. 



Heat, like light, is physically a form of vibration of the 

 ether. 



1. The temperature sense depends upon the fact, that when 

 heat is withdrawn from the body, a sensation of coolness or 

 cold, and, when heat is added to the body, a sensation of 

 luarmth or hecU is produced. This depends upon the tempera- 

 ture of the body in relation to the surroundings, and not 

 merely on the temperature of the surroundings. If three 

 basins of water are taken, one very hot, one very cold, and one 

 of medium temperature, and if a hand be placed, one in the 

 very hot and one in the very cold water for a short time, 

 and then transferred to the basin with water at a medium 

 temperature, the water will feel hot to the hand that has 

 been in the cold water and cold to the hand that has been 

 in the hot water (Practical Physiology). 



2. The rate at which heat is abstracted or added is the 

 governing factor in causing the sensation ; a sudden change 

 of temperature stimulates far more powerfully than a slow 

 change. For this reason the thermal conductivity of sub- 

 stances in contact with the skin has an influence upon the 

 sensation. If a piece of iron and a piece of flannel laid side 

 by side be touched, the first will feel cold, the second will 

 not. This is because the former has high thermal con- 

 ductivity, the latter has not, and thus the former abstracts 

 heat more rapidly than the latter. When the skin of a horse 

 is covered by sweat, heat is rapidly abstracted and shivering 

 may be produced (p. 2G7). 



3. Certain parts of the skin are stimulated by the with- 

 drawal of heat, and their stimulation is accompanied bv 

 sensations of cold, while others are stimulated by the addition 



