i io VETERINARY PHYSIOLOGY 



But if the wheel is made to rotate more and more rapidly, the 

 separate sensations are no longer felt, but a continuous sense 

 of contact is experienced. (Practical Physiology.} This in- 

 dicates that, if stimuli follow one another sufficiently rapidly, 

 the sensations produced are fused. From this it is obvious that 

 the sensation lasts longer than the stimulus the contact. 



The paths of conduction to and in the central nervous system 

 and the position of the centre in the brain are considered on 

 p. 205. 



(b) FOR ADDITION AND WITHDRAWAL OF HEAT 

 Thermal Sense 



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

 ether. The temperature sense depends upon the fact that 

 when heat is withdrawn from the body we have one kind of 

 sensation which we call cold, and when heat is added to our 

 body another sensation which we call hot. This depends upon 

 the temperature of our body in relationship to the surround- 

 ings,' and not merely on the temperature of surrounding 

 bodies. 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 upon the 

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

 that has been in the hot water. (Practical Physiology.} 



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 side by 

 side be touched, the first will feel cold, the second will not, 

 because the former has high thermal conductivity, the latter 

 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, and their stimulation is accompanied by sensations 

 of cold, while others are stimulated by the addition of heat 



