THE FROG-HEART 7 1 



a thread under the sinus, and tighten it round the sino-auricular 

 junction. The sinus continues to beat as before (count the rate), but 

 the auricle and ventricle usually come to a standstill in diastole. Such 

 a heart is termed a Stannius heart. 



Gently prick auricle or ventricle, and notice that each stimulation 

 is followed by a contraction starting from the point of stimulation and 

 spreading to all parts of auricles and ventricle. They are therefore 

 not inhibited (prevented from contracting), but have ceased to contract 

 owing to their being cut off, by the ligature, from the sinus, in which 

 the contractions normally begin. 



Now tie a second ligature round the auriculo-ventricular junction. 

 The ventricle usually gives three or four beats, and then both it and 

 the auricle again come to a standstill. Either can, however, be made 

 to beat by artificial stimulus (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 are also 

 spontaneously contractile ; even small pieces can be observed to beat 

 rhythmically. 



Peculiarities of cardiac contraction. Apart from its regular 

 spontaneous rhythm the heart muscle shows certain phenomena, which 

 are probably not peculiar to it, but are much more strikingly exhibited 

 than by skeletal muscle. To investigate these it should be used as a 

 muscle, the spontaneous contractions being eliminated as follows : 



Apply a ligature at the base of the ventricle ; cut out the heart 

 and attach the apex by means of a very fine hook and thread to a very 

 light muscle lever (such as is shown in Fig. 62), the preparation being 

 fixed by a pin passed through the ligatured base. Or the heart may, if 

 desired, be left in situ as shown in the figure. Be very careful not to 

 injure the preparation. Contractions are recorded by the lever in the 

 same way as those of any other muscle. Use a very slow drum. As a 

 rule there are no spontaneous contractions of the ventricle, but it 

 responds to a stimulus applied to any part of its surface. 



Allow a pair of electrodes, connected with an induction coil, to 

 touch the base of the quiescent heart. They must be fixed (this can be 

 done with plasticine), not held in the hand. The following experiments 

 may now be made : 



1. Staircase phenomenon (see p. 46). Use the above "Stannius" 



