272 PRACTICAL PHYSIOLOGY. [LV. 



a small electro-magnetic signal. This will begin to vibrate when 

 the primary circuit is closed, and mark the period of stimulation as 

 a white patch on the black surface. Make its point record exactly 

 under the heart-lever. Take a time-tracing in seconds. 



(b.) After a pause the beat begins, the contraction travelling 

 as a wave from sinus, through auricles to ventricle. 



(f.) Stimulate the auricles. During inhibition the sinus beats, 

 but the auricles and ventricle do not, because the excitabihty of the 

 auricles is so lowered that they do not propagate the excitatory 

 process. 



(d.) Stimulate the ventricle mechanically, the heart beats in the 

 reverse order from ventricle, auricles to sinus. 



FlO. 194. — Arrest of the Frog's Heart- Beat by Electrical Stimulation of the Crescent. 

 See. Time in seconds ; H. Heart-beats ; S. Stimulation. 



5. Seat of the Motor Centres. 



(a.) Expose a pithed frog's heart, cut out the ventricle with the auricles 

 attached to it, and observe that the heart continues to beat. Divide the 

 ventricle vertically by two parallel cuts into three portions. The middle 

 portion contains the auricular septum, in which lie ganglionic cells. It con- 

 tinues to beat while the right and left lateral parts do not beat spontaneously, 

 but respond by means of a single contraction if they are stimulated. 



6. Latent Period of Cardiac Muscle (Cardiac Delay). — This is 

 ascertained in the same way as in a skeletal muscle, but there is this 

 difference, the heart beats rhythmically while the skeletal muscle 

 is at rest until excited. Therefore the heart-beat must be brought 

 to a standstill. This can be done by a Stannius hgature. 



{a.) Arrange an induction coil to give single shocks, putting in 

 the primary coil an electro-magnet which records its movement on 

 a slow-revolving drum. This will indicate the moment of stimu- 

 lation. 



(h.) Expose the heart in a pithed frog, " Stannius " its heart 

 (Lesson LV.). This will arrest its beat. Then tie a silk thread to 

 the apex of the ventricle, and attach the thread to a Gaskell's heart- 

 lever. Arrange the heart-lever so that it records on a drum exactly 

 above the electro-magnet. 



