PRACTICAL EXERCISES 



175 



applied to the heart, but it could be shown by a separate experiment 

 that atropia by itself has the same effect on the vagus endings.) 

 (P. 141.) 



10. Stannius' Experiment. Pith a frog. Expose the heart in the 

 way described under 2 (p. 168). Ligature the fraenum with a fine silk 

 thread, and use the thread to manipulate the heart. With a curved 

 needle pass a moistened silk thread between the aorta and the 

 superior vena cava, and tie it round the junction of the sinus and 

 right auricle (Fig. 58). The auricles and ventricle stop beating as soon 

 as the ligature is tightened. The sinus venosus goes on beating. Now 

 separate the ventricle from the rest of the heart by an incision through 

 the auriculo-ventricular groove, or tie a second ligature in the groove. 

 The ventricle begins to beat again, the auricle remaining quiescent 

 in diastole (p. 142). Occasionally both auricle and ventricle, or only 

 the auricle, may begin to beat. 



11. Stimulation of Cardiac Sympathetic Fibres in the Frog. 

 (i) In the v ago-sympathetic after the inhibitory fibres have been cutout by 

 atropia. Arrange everything as in 7 (p. 173). Assure yourself, by 



FIG. 65. ARRANGEMENT OF INDUCTION MACHINE FOR TETANUS. 



B, battery ; K, simple key ; P, primary coil ; S, secondary coil ; A, C, binding 

 screws to be connected with battery for single shocks ; F, G, binding screws for 

 tetanizing current ; N. Neef's hammer ; D, short-circuiting key in secondary ; 

 E, electrodes. D and E are drawn to a much larger scale than the rest of the figure. 



stimulating the vagus, that it inhibits the heart, and take a tracing 

 during stimulation. Then paint a dilute solution of atropia on the 

 sinus. Stimulation of the vagus, which is really the vago-sympathetic 

 (see Fig. 64), will now cause, not inhibition, but augmentation (increase 

 in rate or force, or both), since the endings of the inhibitory fibres 

 have been paralyzed by atropia. The strength of the stimulating 

 current required to bring out a typical augmentor effect is greater than 

 that needed to stimulate the inhibitory fibres. Take a tracing to show 

 augmentation produced by stimulating the nerve. 



(2) By dirett stimulation of the cervical sympathetic. Make the 

 same arrangements as in n (i), but, instead of isolating the vagus, 

 dissect out the sympathetic on one side in the manner described in 

 6 ( 2 ) (P- I 7 2 )y and do not apply atropia to the heart. Lay the upper . 



