PRACTICAL PHYSIOLOGY 65 



tation of the sympathetic fibres which have joined 

 the vagus near the skull and are running with the 

 cardiac branches to the heart ; with stronger stimu- 

 lation the heart will beat more slowly and less 

 vigorously or may stop altogether. 



5. Place a drop or two of a weak solution (0.2 

 per cent.) of nicotin upon the heart, and after a 

 minute or two again stimulate the vagus ; no 

 effect should be obtained, since nicotin blocks the 

 junction of the nerve fibres with the distributing 

 nerve cells within the heart. Wash away the 

 nicotin with salt solution, and the effect will re- 

 turn after a time. 



6. Disconnect the electrodes which are at- 

 tached to the skull from the commutator, and use 

 them to stimulate the heart itself 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 other- 

 wise.) The heart again comes to a standstill in 

 diastole. Record this effect also. 



Notice that in each case there is an after-effect 

 of a nature contrary to the immediate effect. 



7. Drop a very small quantity of dilute solution 

 of muscarin upon the sinus, recording the effect 

 produced upon the rate and force of the beat. 

 After a short time the heart will probably come 

 to a standstill in diastole. Now wash away the 

 muscarin with two or three drops of solution of 

 atropin sulphate (i in 300). Notice the gradual 

 restoration of the rate and force of the beats. 

 Notice further that no inhibition can now be 



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