INNERVATION OF HEART AND BLOOD-VESSELS 563 



line of junction of the sinus venosus and right 

 auricle. Tie a ligature around the heart exactly 

 at this line, passing the thread beneath the aortse, 

 so that they shall not be included in the ligature. 



The auricles and ventricle cease to beat, for a 

 time at least, while the sinus venosus continues 

 with unaltered rhythm. (The result is usually 

 ascribed to inhibition, from the mechanical stim- 

 ulation of the intracardiac inhibitory mechanism. 

 If the ventricle begins spontaneously to beat, as 

 may happen if the ligature is not accurately 

 placed, tie a second ligature around the junction 

 of sinus and auricle.) 



Action of Nicotine. Apply nicotine solution 

 (0.2 per cent) to the ventricle. After a few 

 minutes, stimulate the trunk of the vagus nerve. 

 No curve need be written. 



The heart is not inhibited. 



Now lift the heart with a glass rod, and stimu- 

 late the intracardiac inhibitory nerves. 



The heart is inhibited. Nicotine paralyzes 

 some inhibitory mechanism between the vagus 

 and the intracardiac inhibitory nerves. But it is 

 known that nicotine does not paralyze nerve 

 trunks. Hence it is probable that the cardiac 

 inhibitory fibres do not pass to the cardiac muscle 

 directly, but end in contact with nerve cells, 

 which take up the impulse and transmit it 



