ADDENDA. 



According to Head, Rivers and Sherren (Brain, 1905) three 

 distinct mechanisms operate in the production of common sensa- 

 tion, one concerned with deep sensibility and two with superficial 

 sensibility. The mechanism of deep sensibility includes the nerves 

 supplying muscles, tendons, joints and ligaments. Though it 

 may be affected by environment, it is chiefly acted upon by stimuli 

 originating within the organism, such as pressure (resulting from 

 weight and inertia) and tension. It may give rise to painful 

 sensations, but its principal concern is with muscular tonicity 

 and equilibrium. The nerves of the deep mechanism constitute 

 the " proprio-ceptors " of Sherrington. They are so named be- 

 cause their stimuli are furnished chiefly by the organism itself 

 (Brain, 1906). The mechanisms of superficial (cutaneous) sen- 

 sibility are called by Head, Rivers and Sherren the Protopathic 

 and the Epicritic. The protopathic mechanism responds to pain- 

 ful cutaneous stimuli and to extremes of heat and cold. It may 

 set up reflexes without arousing consciousness. Cutaneous local- 

 ization is no part of its function. The mechanism of epicritic 

 sensibility is adapted to mild stimuli, to light touch, to warmth, 

 to coolness. By it the localization of cutaneous stimuli is accom- 

 plished and its activity usually involves consciousness. It may 

 also produce reflexes through the efferent nerves. Sherrington 

 speaks of these two mechanisms as the " extero-ceptors " and 

 'gives the name "intero-ceptors" to the nerves of the interior sur- 

 faces, those of the alimentary tract, etc. The "intero-ceptors" 

 are especially adapted to chemical stimuli and give rise to diges- 

 tive reflexes; but they may also produce pain and temperature 

 sensations, and may respond to stimuli occasioned by tension and 

 pressure. See pages 365 and 377. 



