ADVANCED EXPERIMENTAL PHYSIOLOGY 223 



Mechanical stimulation with a metal rod warmed to the same tempera- 

 ture as that of the skin of the hand will give rise to sensations of 

 touch, temperature, or pain according to the area stimulated. Muller's 

 law is thus demonstrated in the case of these sensations. 



There is some doubt whether there are specific nerves for painful 

 sensations ; it may be that excessive stimulation of any sensory nerve 

 causes pain. 



CHAPTER LXIV (Advanced). 



CUTANEOUS SENSATIONS. SENSATIONS DERIVED FROM 

 MOVEMENTS. 



THE sensations which arise from stimulation of the sensory nerves 

 of the skin include four separate qualities. These cutaneous sensations 

 may therefore be divided into (a) sensations of pressure, (b) sensations 

 of warmth, (c) sensations of cold, (d) sensations of pain. 



On the other hand, surfaces in the interior of the body, such as the 

 membranes of the alimentary canal, etc., furnish only sensations of 

 pain, which are bound up with sensations referred to the skin, referred 

 pain. The defensive mechanism connected with pain has been desig- 

 nated protopathic, and is of a lower developmental type than the 

 mechanism which furnishes us with the means of making the fine dis- 

 criminations of touch and temperature. This more highly specialised 

 mechanism has been designated epicritic. It has not yet been found 

 possible to definitely connect these different forms of sensations with 

 different varieties of sensory nerve endings. 



By simple experiments it is easy to show that the arrangement of 

 the machinery which originates these sensations is not regular and 

 continuous, an important feature being the punctiform distribution of 

 the cutaneous senses. 



I. METHODS ADAPTED TO ASCERTAINING THE DISTRIBUTIONS 

 OF THE CUTANEOUS SENSES. 



a. The sense of pressure. It is necessary to distinguish between 

 those lighter pressures which affect practically only the epidermis and 

 the heavier pressures which can excite the subcutaneous tissues. Only 

 the first of these can be strictly considered to be concerned in the 

 sensation of touch. 



