64 EXPERIMENTAL PHYSIOLOGY. 



centic line at the sino-auricular junction and the auricles. 

 What is the sequence in the beat? Record the normal heart 

 beat by connecting the tip of the ventricle to a heart lever. 



Prepare for dissection of the vagus by stretching the neck 

 tissues over a large glass rod inserted into the oesophagus. 

 On clearing away the connective tissue near the angle of the 

 jaw three nerves can be found running from below the angle 

 diagonally downward (Fig. 5). The upper one glosso-pharyn- 

 geal, soon turns and passes forward; the lower one also passes 

 forward in a similar manner. Lying between these nerves 

 before they bend upward can be found the third nerve or vago- 

 sympathetic lying under the edge of a fine muscle which connects 

 the angle of the jaw with the hyoid bone. Stimulate the vagus 



FIG. 17. Effect of vagus stimulation upon the heart of the turtle. Note the increased 

 amplitude following the inhibition. 



to make sure that the nerve is effective in modifying the heart 

 rate (Fig. 17). Use the tetanizing current, beginning with a 

 weak current and gradually increasing if there is no effect. 

 Bear in mind that the sympathetic is being stimulated at the 

 same time and therefore its stimulation may counteract the 

 effects of vagus stimulation. That is the reason that it is 

 necessary to find a current which will affect the vagus more 

 than the sympathetic. Obtain a record of the contracting heart 

 before and after vagal stimulation, using the suspension method 

 as shown in Fig. 18. Use the magnetic signal to mark the 

 time of stimulation. If you fail to obtain effects from one 

 vagus, try the other. 



