LESSON XIV 

 THE HEART (Continued) 



INHIBITION AND ACCELERATION OF THE SIMPLE HEART. ACTION OF 

 NICOTIN, ATROPIN, AND MUSCARIN 



1. Inhibition of the Heart. Apply a ligature tightly to the neck of 

 an etherized turtle and destroy the brain by pithing. Remove the 

 ventral shield or plastron and clip away the projecting angles of the 

 shoulder-blades. Arrange the inductorium for stimulation with a 

 tetanic current of medium strength. Hold the extended neck of the 

 turtle in place and carefully isolate the vagus nerve on each side. Place 

 each in a loose ligature. Open the pericardial sac and connect the 

 apical band of connective tissue with the writing lever. Adjust the 



SM-- j 



FIG. 53. COURSE OF VAGUS NERVE IN FROG. (Stirling.) 



SM, Submentalis; LU, lung; V, vagus; GP, glossopharyngeal ; HS, hypoglossal; L, 

 laryngeal; PH, SH, GH, OH, petro-, sterno-, genio, and omohyoid; HG, hypoglossus; 

 H, heart; BR, brachial plexus. 



writing point of a chronograph underneath the writing point of the 

 lever. Raise the left vagus nerve and place it upon the electrodes. 

 Having recorded a number of normal heart-beats, stimulate the afore- 

 said nerve. If the heart is not inhibited, increase the strength of the 

 current, but not excessively, because an electrolysis might then result 

 which would destroy conduction permanently. 



Make a number of these records, stimulating each time for a few 

 seconds. During what period of the cardiac cycle is the heart arrested? 



85 



