74 EXPERIMENTAL PHYSIOLOGY 



to excitation of the sympathetic fibres which have joined the vagus 

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

 with stronger stimulation the heart will beat more slowly and less 

 vigorously or may stop altogether. 



5. Place one drop of a weak solution (0'2 per cent.) of nicotine upon the sinus ; 

 the effect of this is at first to slow the heart, because the nerve cells to which 

 the vagus fibres are distributed are stimulated by the drug ; subsequently they 

 are paralysed, and the heart resumes its normal rate. After a short interval 

 stimulate the vagus. No effect should be obtained, since nicotine blocks the 

 junction of its nerve fibres with the distributing nerve cells within the heart. 

 Wash away the nicotine with salt solution ; the effect will return after a time. 



6. Stimulate the heart at the white line of the sino-auricular junction. (The 

 electrodes must not be held in the hand, but must be fixed in position by a pin 

 through their cork or by plasticine.) The heart comes to a standstill in diastole. 

 Record this effect by a continuous tracing. This standstill is not due to the 

 stimulation of an inhibitory centre, but to the fact that the inhibitory fibres are 

 close to the surface at this place. 



Notice that in each case of acceleration or retardation of the pulse-rate there 

 is an after-effect of a nature contrary to the immediate effect. 



7. Place a single drop of dilute solution of muscarine upon the sinus, recording 

 the effect produced upon the rate and force of the heart, which will soon come 

 to a standstill in diastole. Now wash away the muscarine with two or three 

 drops of solution of atropine sulphate (1 in 300). Notice the gradual restoration 

 of the rate and force of the beats. Notice further that no inhibition can now be 

 produced on stimulating either the vagus or the sino-auricular junction. There 

 may, however, be acceleration, from stimulation of the sympathetic fibres which 

 are running to the heart in the cardiac branch of the vagus. 





