442 



ON IRRITANTS AND COUNTER-IRRITANTS. 



tlie vaso-motor nerves for the ear pass from the medulla along 

 the cervical sympathetic to the arteries, as shown in Fig. 154. 

 When the cervical sympathetic is cut, the vaso-motor centre 

 can no longer exercise any influence on the arteries of the ear, 

 and they dilate as already desert b 3d. 



Fig. 1C4. — Diagram of the Vaso-motor Nerves of the Rabbit's Ear. 



i/'^SO MO TOR CENTRE 

 //V MEDULLA 



Vasomotor ^/erves 



for tme {/ess el s 



of the ear 



VaSO MOTOR NERVES 

 FOR OTHER PARTS 

 OF THE BODY 



But there is another way of making them dilate, viz., by caus- 

 ing the vaso-motor centre to cease to exert its usual influence 

 over these vessels, instead of destroying its means of communi- 

 cation with them by cutting the vaso-motor nerves. This can 

 be done by irritating a sensory nerve to the ear. The irritation 

 is conveyed up to the vaso-motor centre, and seems to arrest its 

 action over the vessels supplying the part whence the sensation 

 has come, so that they dilate, and the ear gets red and hot 

 This dilatation, in consequence of irritation applied to a sensory 



