STIMULATION OF THE CARDIAC VAGUS. 



'217 



2. Stimulation of the Cardiac Vagus. 



(a.) Adjust a heart-lever so as to record the contractions 

 of the heart on a revolving drum moving at a very slow rate. 



(b.) Place well-insulated electrodes under the trunk of 

 the vagus, stimulate it with an interrupted current, and 

 observe that the whole of the heart is arrested in diastole. 

 Although the faradisation is continued the heart recom- 

 mences beating. The arrest, or period of inhibition, is manifest 

 in the curve by the lever recording merely a straight line. If 



Heart Beat. 



Time in Sees. 



Stimulation 



Fig. 99. Vagus Curve of Frog's Heart. 



the laryngeal muscles contract, and thereby affect the posi- 

 tion of the heart, divide the laryngeal branch of the vagus. 



(c.) Note that when the heart begins to beat again, the 

 beats are small at first and gradually rise to normal. In 

 some instances, however, they are more vigorous and 

 quicker (Fig. 99). 



3. Determine the Latent Period. For this purpose a time- 

 marker and an arrangement to indicate when the stimulus is 

 thrown into the nerve are required. 



(a.) Arrange the heart-lever as before, and adjust a time- 

 marker to write exactly under the hearfc-lever. 



(b.) Arrange an induction machine for an interrupted 

 current, and keep Neefs hammer vibrating. Into the 

 secondary circuit introduce an electro-magnet with a writing- 

 lever attached to it; so adjust the electro-magnet that 

 its writing-style writes exactly under the heart-lever, and 

 arrange that when the writing-style on the electro-magnet 

 is depressed e.g., by means of a weight the secondary 

 circuit is short-circuited, so that no stimulus is sent along 

 the electrodes under the trunk of the vagus. 



