122 EXPERIMENTAL PHYSIOLOGY 



Experiment 1. — Study the Influence of the vagus upon the heart. Pre- 

 pare the coil for giving tetanising shocks. Dissect out both vagi. It is 

 best to cut the laryngeal branch of each vagus because if this be left intact 

 the muscles contract when the nerve is stimulated and acting upon the 

 pericardium, pull down the base of the heart, and so alter the level of 

 the tracing. Attach the heart to the recording lever by the suspension 

 method, fig. 83. If the heart be beating very slowly apply a little warm 

 normal saline solution (at 20° C.) by a pipette until the beat is quickened. 

 Remove the secondary coil to some distance from the primary. The 

 electrodes, fig. 87, should have their ends bent up into a hook, and the 

 outer sides of the wires may be further insulated by a little sealing-wax to 

 prevent escape of the current. Hook up one vagus on to the electrodes. 

 Now take a piece of tracing, the drum rotating about 1 to 2 mm. per second, 

 and with a seconds time tracing recorded directly beneath. After about ten 

 contractions open the key in the secondary circuit, so that the nerve may be 

 stimulated, and mark on the tracing the instant at which the stimulus was 

 applied. If no change occur close the key, marking the instant at which 

 the stimulation was stopped. Next move the coil 2 cm. nearer the primary, 

 and repeat the stimulation as before. Repeat several times, gradually in- 

 creasingthe strength of the stimulus until one or two tracings showing complete 

 inhibition have been recorded. Time must be allowed after stimulation is 

 stopped to record the full series of changes occurring after stimulation. 

 Repeat the series, stimulating the vagus of the opposite side. 



It must be remembered that in this experiment we are stimulating 

 both sets of cardiac fibres, sympathetic as well as vagal. 



In fig. 100 the result is shown. The laryngeal branch was not cut, 

 and we see that on each stimulation the general level of the record 

 was lowered. When the secondary was at 19 cm. (tracing 1) there 

 was a very slight slowing effect. One evidence of this is to be seen in 

 the auricular tracing. Before stimulation the auricle began to contract 

 before relaxation of the ventricle was complete. During the stimula- 

 tion, however, the time interval between two successive beats is very 

 slightly increased, and this allowed relaxation of the ventricle to be com- 

 pleted before the next auricular contraction commenced. In tracing 2 

 with an increase in strength of stimulation there is a clear diminution 

 of rate. Note that the effect does not occur instantaneously. There is a 

 gradual production of the slowing lasting over four beats. The rate 

 before stimulation was 38 per minute ; towards the end of the time of 

 stimulation it was 30 per minute. A second change, occurring as a 

 result of the stimulation, is an increase in the force of each beat as 

 judged by the height of the tracing. In the measurement of these trac- 

 ings, where we are examining the heights of contraction, it must be re- 

 membered to measure from the point where the ventricular contraction 

 begins to the highest point reached. It will not do to take the total 

 height of the tracing as the measure of force, because at times a con- 

 siderable amount of the rise may be due to auricular contraction with 

 ventricular superadded ; at other parts of the tracing it may be 

 purely ventricular contraction. Examine in this connection fig. 102 



