THE FROG-HEART 



67 



it and the auricle again come to a standstill. Either can, however, 

 be made to beat by artificial stimulation (prick, electric shock). 



After a certain lapse of time (sometimes very soon after the appli- 

 cation of the first ligature) the auricle and ventricle may recommence beating 

 regularly and rhythmically, but it will be found on counting the rate that it is 

 never as fast as that of the sinus. If the auricle is cut off from the ventricle, 

 as by the second Stannius ligature, all three parts may ultimately be found 

 beating spontaneously, but it will always be noticed that the rate of the sinus 

 is the fastest, that of the auricles next, and that of the ventricle the slowest. 

 The bulbus aortse is also spontaneously contractile ; even small pieces can be 

 observed to beat rhythmically. 



Peculiarities of cardiac contraction. Apart from its regular 

 rhythm the heart muscle shows certain peculiarities as compared with 



FIG. 60. PROG CARDIOGRAPH. /, PROG ; /;, HEART ; I, LEVER. 



skeletal muscle. To investigate these, apply a ligature at the base 

 of the ventricle ; cut out the heart and attach the apex of the ventricle 

 by means of a fine hook or serafin and thread to a muscle lever (such 

 as is shown in fig. 60), the preparation being fixed by a pin 

 passed through the ligatured base. The heart may, if desired, be left 

 in situ as shown in the figure. Contractions are recorded by the lever 

 in the same way as those of any other muscle, but on a very slow 

 drum. Be very careful not to injure the preparation. As a rule there 

 are no spontaneous contractions, but the ventricle responds to the 

 least stimulus applied to any part of its surface. The following 

 experiments are to be made with this preparation : 



1. "All or none " contraction. Allow a pair of electrodes, con- 

 nected with an induction coil, to touch the base of the quiescent 

 heart. They must be fixed (this can be done with plasticine), not 



