ELEMENTARY EXPERIMENTAL PHYSIOLOGY 103 



The Extra-systole and Compensatory Pause. Excite with a single * 

 induction shock a rhythmically beating heart. The heart is recorded 

 as in Fig. 90 or 93. An extra contraction excited during the diastolic 



FIG. 99.- Effect of tetanising the Stanniused heart. The curve should be read from 

 left to right. The time is marked in seconds. The third line shows the period of 

 stimulation. (L.H.) 



period of the rhythmically beating heart is followed by a compensatory 

 pause. Note that the heart does not respond when excited during 

 systole the refractory period (Fig. 150). 



This period of inexcitability is seen in skeletal muscle (p. 42), but 

 is of much shorter duration than the refractory period of the heart. 

 The difference probably depends upon a slower metabolism in the 

 cardiac muscle. 



CHAPTER XXVIII. 

 THE CARDIAC NERVES AND GANGLIA. 



The Intra-cardiac Ganglia, and Nerves. The vago-sympathetic 

 nerves enter the sinus with the superior venae cavae, and form a plexus 

 there which contains many ganglion cells (Ramak's ganglion). The 

 nerves pass on to enter the auricular septum, which also contains 

 ganglion cells (v. Bezold's ganglion). Leaving the septum the nerves 

 enter the auriculo-ventricular junction, where third groups of ganglion 

 cells lie (Bidder's ganglion). 



To see these structures (Fig. 100), forcibly inject the living heart 

 with osmic acid (1 % sol.), passing the needle of the hypodermic syringe 

 into the auricle. The osmic acid almost instantaneously fixes the heart 

 in distension. Cut out the heart and place it in a watch-glass of 1 % 

 osmic. After 5 minutes open the auricles under water, look for the 



