104 VETERINARY PHYSIOLOGY 



of heat and give rise to a sense of warmth. This may be 

 demonstrated 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. Such cold and hot spots have 

 been excised and examined microscopicall}-, but no 

 special receptor mechanisms have been found. In all 

 probability, special developments of the intercellular plexus in 

 the epidermis form the receptors (Practical Physiology). 



4. The temperature sense is independent of the tactile 

 sense, and the changes set up travel in separate fibres in the 

 cord (p. 112). The one sense may be lost and the other 

 retained. 



4. Special Cutaneous Receptors. 



In certain regions of the skin special receptor organs 

 exist, stimulation of which excites reflexes in connection with 

 the sexual and other special functions. In the anal canal 

 tactile and thermal receptors exist, but distension gives rise 

 to a special sensation — the desire to defsecate. 



Von Frey's observations on man indicate that in the skin 

 there are at least four receptor mechanisms, stimulated separ- 

 ately by pain, by touch, by the withdrawal of heat, and by the 

 addition of heat. He finds that certain parts of the 

 surface do not respond to all three kinds of stimuli, but 

 only to one or two. Thus, pain alone can be produced 

 from the cornea, tooth pulp and dentine, and the parietal 

 pleura ; pain and temperature sensations, but not sense of 

 touch, from the edge of the cornea and conjunctiva ; and 

 touch and temperature sensations alone from certain patches 

 inside the mouth. 



When a severed nerve begins to unite and regenerate (p. 77), 

 an imperfect return of all these types of sensation occurs, but 

 only after complete regeneration are the senses completely 

 restored. 



In addition to the interoceptive and exteroceptive 

 receptors of the body, other receptors are found throughout 



