LABORATORY WORK 217 



is a pause before the foot is withdrawn, and that this is longer, the 

 weaker the acid. 



Find a strength of acid that invariably results in withdrawal 

 of the foot after a moderate time. Then pinch firmly the opposite 

 foot at the same time as the acid is applied. The reflex is stopped 

 or takes much longer to show itself (inhibition}. 



Inhibition 



That an action in progress can be stopped by nervous influences 

 is most clearly shown by stimulating the vagus nerve, and thus 

 stopping the beats of the heart. It is somewhat difficult to dissect 

 out this nerve in the frog, but it may be stimulated at its origin 

 from the bulb in the following way : 



Kill a frog by cutting off the brain with scissors just behind 

 the eyes. Then cut through the spinal column between the 

 shoulder blades and destroy the posterior part with a pin. The 

 bulb is thus isolated, with the vagus nerves passing from it to the 

 heart. 



Expose the heart and connect the apex of the ventricle to the 

 lever as on p. 207 above. Put electrodes into the exposed surface 

 of the bulb where the front part of the head has been cut off, 

 fixing them to the cork by means of plasticine. Stimulate at first 

 with weak currents, increasing the strength until the heart stops. 



The fact may also be shown by stimulating the vagus nerves 

 in their course from the sinus to the auricles. The electrodes are 

 placed on the white crescentic line marking the junction of these 

 chambers. Only a weak stimulus must be used, otherwise the 

 muscle itself is stimulated to rapid contractions. 



