NEUKO-MUSCULAK MECHANISM 



in 



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. 

 (Practical Physiology.) 



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. 



Much light has been thrown by Head on the nature of the 

 nervous mechanism connected with cutaneous sensibility in 

 man and probably in the lower animals. He had the radial and 

 external cutaneous nerves in his own arm divided below the 

 elbow, and he thus severed all fibres from the skin over an area 

 on the outside of the hand and fore-arm. 



(1) Immediately after the operation he found that the pressure 

 sense was not lost, and that a touch with a pencil was felt and 

 well localised. True tactile sense, as tested by touching with a 

 soft substance like cotton wool, and the sense of pain as tested 

 by the prick with a pin, were lost. Ulcers tended to form 

 over the paralysed area. 



(2) After seven weeks he found that a prick with a pin could 

 be felt as a painful sensation, not well localised and radiating 

 widely. Differences of temperature between ice-cold water and 

 water at 50 C. could be appreciated, and the cold and hot spots 

 were sharply defined and reacted as cold to water at 24 C. and 

 hot to everything above 38 or 40 C. The sensation radiated 

 widely and was not graduated, the intensity depending upon the 

 number of spots stimulated, large surfaces at 25 C. giving a 

 more marked sensation of cold than small surfaces at C. 



By the end of 200 days this condition was completely re- 

 stored over the whole area, and ulcers no longer tended to form. 

 He considers that the nerve structures involved constitute the 

 great reflex mechanism presiding over the nutrition of the skin. 

 In action it produces qualitative not quantitative changes in the 

 consciousness. He terms it Protopathie Sensibility. 



(3) For more than a year the area remained insensitive to light 

 touch with cotton wool, then gradually it became sensitive to 



