128 EXPERIMENTAL PHYSIOLOGY 



the conduction of the contraction from auricle to ventricle, so that the 

 auricle has quite relaxed before the ventricular systole begins. Fur- 

 ther, as the rate of beating is faster, it is seen that a contraction of the 

 auricle commences before the ventricular contraction is complete. 



iv. A gradual return to the normal state after stimulation has 

 ceased. The acceleration gradually disappears. The force of the 

 beat at once begins to decline until it reaches the original amount. 

 A change in conduction occurs in the reverse direction to that found 

 during stimulation. Eate of conduction is increased, and ventricular 

 systole commences soon after the auricular contraction reaches its 

 maximum. The result is that there is more summation of auricular 

 contraction to ventricular, with the result that the total amplitude of the 

 trace is greater than during stimulation, though each individually is 

 less. This increased rate of conduction gradually dies away and 

 returns to the original value. 



These results should be directly contrasted with those found on 

 vagal stimulation (p. 122), where they are seen to be in exactly the 

 reverse direction. 



In ii a similar effect is found in all particulars, differences being 

 only of degree. Eate is increased from 16 to 32 ; height from 9 mm. 

 to 11*5 mm. ; and change of sequence is of the same character. 



Tracing in is from a less exhausted heart, and shows the changes 

 very clearly. 



Eate before stimulation was 25, during stimulation 42 per minute. 



Height of ventricular contraction before stimulation was 9"5, during 

 stimulation 14 mm. 



Change of sequence is at first as in previous tracings, but this is 

 then followed by another alteration in which the auricular contraction 

 commences later in the ventricular relaxation, though conduction is 

 still delayed as previously. After stimulation ceases the same stages 

 are passed through as were previously described, and to a very marked 

 degree. 



The Second Stannius Ligature. — In a previous series of experi- 

 ments we have seen that a ligature applied between the sinus and 

 auricles leads to a standstill of the auricles and ventricle in diastole, 

 while the sinus continues beating with unaltered rhythm. 



Experiment 4. — Complete the Stannius experiment by apptying a second 

 ligature at the junction of the auricles with the ventricle. 



It will usually be found that the auricle still remains quiescent, 

 but that the ventricle begins to beat. Count the rate at which 

 sinus and ventricle beat. It will be found that the ventricle is 

 beating at a slower rate than the sinus. This is the rule, but 

 occasionally it is found that the auricle only begins to beat, or 

 again that both start beating. As to the meaning to be given to this 



