214 EXPERIMENTAL PHYSIOLOGY. 



LESSON XLVI. 



STANNIUS'S EXPERIMENT AND INTRA- 

 CARDIAC INHIBITORY MOTOR CENTRE. 



1. Stannius's Experiment. Pith a frog, and expose its heart 

 in the usual way. 



(a.) With a seeker clear the two aortue from the auricles, 

 and with an aneurism needle pass a moist stout thread be- 

 tween the two aortae and the superior vense cavse, turn up 

 the apex of the heart, divide the frsenum, and raise up the 

 whole heart to expose its posterior surface, and the crescent 

 or line of junction of the sinus venosus and the right 

 auricle. Bring the two ends of the thread ligature round the 

 heart call this for convenience No. 1 ligature tie them, 

 and tighten the ligature just over the "crescent," so as to 

 constrict the line of junction of the sinus venosus with the 

 right auricle. Before tightening the ligature, observe that 

 the heart is beating freely. On tightening the ligature, the 

 auricles and ventricle cease to beat, and remain in a state of 

 relaxation, while the sinus venosus continues to beat at the 

 same rate as before. After a time, if left to itself, the ven- 

 tricle begins to beat, but with an altered rhythm. If the 

 relaxed ventricle be pricked, it executes a single contraction. 



(b.) When the heart is still relaxed, take a second 

 ligature (No. 2), and preferably of a different colour to 

 distinguish it from No. 1, place it round the heart, and 

 tighten it over the auriculo-ventricular groove, so as to 

 separate the ventricle from the auricles. Immediately the 

 ventricle begins to beat again, while the auricles remain 

 relaxed or in diastole. 



(c.) Instead of applying No. 2 ligature, the ventricle may 

 be cut off from the auricles by means of a pair of scissors. 

 Immediately after it is amputated, the ventricle begins to 

 beat. 



2. Intra- Cardiac Inhibitory Centre. 



^ (a.) Expose the heart in a pithed frog, tie a fine silk 

 ligature round the framum, and divide the latter between 



