CARDIAC NERVES OF FROG 



79 



1. Take a normal tracing of the beats during a short period and 

 record by an electro-magnetic signal or by marks made by hand a time- 

 tracing (minutes) upon the drum. This time-tracing is to serve for 

 determining the exact rate per minute of the heart beat under the 

 different circumstances of the experiments. 



2. Whilst this is proceeding, stimulate the medulla oblongata, 

 allowing the result to be recorded continuously at the same level of 



Glossopharyngeal. 



Vagus. 



Laryngeal branch of vagus. 

 Hypoglossal. 



2nd spinal (or brachial) nerve. _ j^ 



Cardiac branch of vagus. \- 



Superior vena cava. 

 Pulmonary branches of vagus. ^N- 



Glottis. 



Left auricle. 

 Bulbus aortse. 

 Right auricle. 



Sinus venosus. 



FIG. 65. Relations of vagus nerve to other structures in the neck and thorax. 

 The ventricle has been drawn over to the left side by a hook and the sinus 

 venosus is thus exposed, x , Line of junction between sinus and auricles. 



the drum. (The excitations must in no case be so strong as to escape 

 to other parts of the preparation.) 



3. Cut both vagus nerves a short distance from the skull and 

 repeat the above stimulation of the medulla oblongata, recording the 

 result at another level of the drum. 



4. Alter the commutator to stimulate the vagus nerve, recording 

 the result at a third level of the drum. With weak stimulation of the 

 vagus in the frog the heart may beat faster and more strongly owing 

 to excitation of the sympathetic fibres which have joined the vagus 

 near the skull and are running with the cardiac branch to the heart ; 



