220 



EXPERIMENTAL PHYSIOLOGY. 



(a.) Pith a frog, expose its heart, and with a fine pipette 

 apply a drop of serum or normal saline containing a trace of 

 muscarin, which rapidly arrests the rhythmical action of the 

 heart, the ventricle being relaxed i.e., in diastole and dis- 

 tended with blood. 



(6.) After a few minutes, with another pipette apply a few 

 drops of a 0*2 per cent, solution of sulphate of atropia in 

 normal saline, the heart gradually again begins to beat 

 rhythmically. 



(r.) Ligature and divide the framum, raise the heart by the 

 ligature, and faradise the crescent or inhibitory centre; the 

 heart is no longer arrested, because the atropin has paralysed 

 the intracardiac inhibitory mechanism. 



(d.) In another frog, arrest the action of the heart with 

 pilocarpin, and then apply atropin to antagonise it, observing 

 that the heart beats again after the action of atropin. 



2. Effect of a Constant Current on the Heart. 



(a.) Expose a pithed frog's heart. Cut out the heart, 

 dividing it below the auriculo-ventricular groove, thus 



obtaining an "apex" prepar- 

 ation which does not beat 

 spontaneously. 



(6.) By means of sealing- 

 wax, fix a cork to a lead base 

 5 cm. square, cover the upper 

 end of the cork with sealing- 

 wax, and thrust through it 

 two wires to serve as elec- 

 trodes, about 4 mm. apart 

 (Fig. 101). Cover the whole 

 with a beaker lined with 

 moist blotting-paper. Place 

 the heart-apex with its base 

 against one electrode, and its 

 apex against the other. 



Fie. 101. Support for Frog's 

 Heart E, Electrodes; H, 

 heart. 



(c.) Arrange two Daniell's cells in circuit, connect them 

 with a key, and to the latter attach the electrodes. Pass a 

 continuous current in the direction of the apex. The heart 



