248 CIRCULATION. 



usually called into action first, the inhibitory fibres exhibiting 

 some delay in manifesting their action. There is much 

 variation in this ; winter frogs show it most. The same 

 result obtains if the trunk of the vagus or its cardiac branch 

 be stimulated. 



The performance of the latter operation is not considered 

 suitable for these exercises, as the branch mentioned is very 

 small and not easy of access. The dissection necessary to find 

 this nerve is given in the appendix. 



23 Effect of atropine and muscarine on vagus action. 

 Next having surrounded the base of the heart with small 

 pieces of blotting paper, bathe its surface around the crescent 

 with tincture of atropine by means of a glass rod. Only 

 apply sufficient to wet the surface itself. In a few minutes 

 repeat the stimulation at the crescent, there will be no 

 response. The vagus is paralysed at its termination. 



Muscarine. Bathe the heart as before with a dilute solution 

 of muscarine. In a little time the heart will come to a stand- 

 still in diastole. This resembles forcible action of the vagus. 

 If too much atropine has been used, the effect will not 

 manifest itself. 



Again bathe the heart with atropine, the heart will presently 

 begin to beat again ; this is taken to prove that the muscarine 

 acted as a stimulant to the vagus. 



Effect of heat and cold on the rate of the heart's 

 contraction. 



Cool some normal saline to 5 C. by means of ice ; with a 

 pipette flow it drop by drop on to the heart whilst the latter 

 is recording its movements. The rate will be slowed. Let 

 the heart recover its previous rate, then drop upon it water 

 which has been heated to 35 C. The rate will be increased. 



