LVI.] CARDIAC VAGUS OF THE FROG. 275 



the position of the heart, divide the laryngeal branches 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. 197). 



3. Latent Period or Delay of Vagus. 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 heart-lever. 



(b.) Arrange an induction coil for repeated shocks, and keep 

 Neef's 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 



Heart Beat. 



Time in Sees. 



Stimulation 



FlQ. 197. Vagus Curve of Frog's Heart. 



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. 



(c.) When all is ready, lift the weight off the electro-magnet, 

 whereby the secondary current is un-short-circuited, the electro- 

 magnet lever rises up, records its movements on the cylinder, and 

 at the same moment the induction shocks are sent through the 

 vagus. Observe that the heart is not arrested immediately, but a 

 certain time elapses the latent period usually about one beat of 

 the heart (1*5 sec.), before the heart is arrested. 



(d.) Short-circuit the secondary current again, and observe how 

 the heart gradually resumes its usual rhythm sinus venosus, 

 auricles, and ventricle. 



(^.) Repeat, (c.) several times, noting that the heart after arrest 

 goes on beating in spite of continued stimulation. 



(/.) An electro-magnet may be introduced into the primary 

 circuit to mark the moment of stimulation just as in Lesson LIV. 6. 



4. Action of the Sympathetic on the Heart of the Frog. 

 (a.) Pith a frog ; or preferably a toad, cut away the lower jaw, and continue 

 the slit from the angle oi the mouth downwards for a short distance. Turn 



