PHYSIOLOGY 



. 2 



more excitable, and 

 an excitatory wave 



FIG. 97. Stannius heart. The first and second 



ligatures (Hedon). 

 1, Auricles ; 2, Sinus ; 3, Ventricle. 



which marks the sino-auricular junction and tie it exactly over this line. 

 The sinus continues to beat, while the auricles and ventricle, after giving 

 a few rapid beats, stand still. The sinus, with its more embryonic type 



of muscle, possesses the greatest 

 power of initiating rhythmic con- 

 traction. The more specialised 

 muscle of the auricles and ven- 

 tricle is 

 conducts 



more rapidly, but is less capable 

 of initiating rhythm. The ex- 

 citatory wave which is started 

 from the sinus is blocked by the ligature; thus the auricles and 

 ventricle cease to beat. Prick the ventricle ; it will respond by a 

 single beat to each stimulus. The Stannius preparation is like a muscle 

 preparation, and can be used to record the contraction of the heart and 

 the latent period. Tie a second ligature just above the auriculo- 

 ventricular groove. Both auricle and ventricle are excited by the 

 ligature and start beating. The rhythm is no longer the same in the 

 three chambers of the heart. The mere contact of the lever or elec- 

 trodes resting on the Stanniused heart sometimes evokes rhythmic 

 contractions. The inhibitory effect of the first ligature has been 

 attributed by some authors to excitation of the vagus nerve. 



The Heart cannot be thrown into Complete Tetanus. Set up a 

 circuit for giving single induction shocks (see Fig. 16, p. 9). Apply 



Pio. 98. Contraction of the frog's heart recorded by the suspension method, 

 effect of tightening the first Stannius ligature at first gently and then firmly, 

 ould be read from right to left. T 



The 



The 



curve should be read from right to left. The time is marked in seconds. (L.H.) 



the electrodes to the Stanniused heart and record the effect of rapidly 

 repeated excitations. The heart gives an incomplete tetanus curve. 

 Owing to the refractory period it cannot be completely tetanised. 



