ELEMENTARY EXPERIMENTAL PHYSIOLOGY 



63 



The periods of contraction and relaxation are prolonged, the frequency 

 greatly diminished. Now fill the beaker with Ringer's fluid at 25 C. 

 The frequency becomes greatly increased, and the period of contraction 

 and relaxation greatly shortened. A temperature of about 35 C. 

 causes diminution of the tone of the heart. The ventricle ceases to 

 follow the auricular rhythm, although it still responds to excitation. 

 At 38 to 43 C. the whole heart ceases to beat, and gradually passes 

 into the condition of heat rigor. The heat contraction, when once 

 fully established, is not set aside by cooling. 



CHAPTER XIV. 



THE STANNIUS HEART. 



The Stannius Heart. The heart of a pithed frog is exposed and a 

 thread is tied to the fraenum which is then cut away from the posterior 

 surface of the pericardium. Pass a ligature under the two aortae and 

 then by means of the thread attached to the fraenum gently pull the 

 heart towards the mouth of the frog. The dorsal aspect of the heart is 

 now readily seen. Draw the ligature round the white crescentic line 

 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 i 

 muscle of the auricles and ven- 

 tricle is more excitable, and 

 conducts an excitatory wave 

 more rapidly, but is less capable 



,,' -i ,-1 T\T^ Fio. 62. Stannius heart. The first and second 



of initiating rhythm. I he ex- ligatures (Hedon). 



Citatory Wave which is Started 1, Auricles; 2, Sinus; 3, Ventricle. 



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 s the lever or elec- 

 trodes resting on the Stanniused heart sometimes evokes rhythmic 



