80 EXPERIMENTAL PHYSIOLOGY 



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 the 

 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 plasticine.) The heart comes to a standstill in 

 diastole. Record this effect in a continuous tracing. This standstill 

 is not due to the stimulation of an inhibitory centre, but to the fact 

 that the inhibitory fibres of the vagus 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 owing to the 

 fact that muscarine stimulates the terminations of the inhibitory fibres^. 

 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, i.e., atropine antagonises muscarine and paralyses the termina- 

 tions of the inhibitory fibres. There may, however, be acceleration, 

 from stimulation of the sympathetic fibres which are running to the 

 heart in the cardiac branch of the vagus. 



