278 



PRACTICAL PHYSIOLOGY. 



[LVIL 



2, Constant Current on the Heart. 



(a.) Pith a frog. Cut out the heart, dividing it below the 

 auricido- ventricular groove, thus obtaining an " apex " preparation 

 which does not beat spontaneously. 



Fig. 199.— Tracing of Heart attached to Gaskell's Lever, arrested by ifnscarine. and 

 Rhytlim restored by Atropine. J/. Muscarine effect ; A. Atropine applied , T. Time 

 in seconds. 



(h.) By means of seahng-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 electrodes, about 4 mm. 

 apart (fig. 200), or by means of sealing-wax fix two fine wires upon 

 an ordinary microscopic glass slide to act as electrodes. Cover the 

 wliole with a beaker lined with moist blotting-paper. Place the 

 heart apex with its base against one electrode, and its apex against 

 the other. 



FlO. 200.— Support for Frog's Heart. 

 E. Electrodes ; H. Heart. 



Fig. 201. — Stairca.se Character 

 of Heart-Beat. 



(c.) Arrange two Dani ell'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 resumes its 

 rhythmical beating, and continues to do so as long as the constant 

 current passes through the living preparation. 



3. The Staircase. 



(a.) To a microscopical glass slide (3x1) fix witli sealing-wax two copper 

 wires in the long axis of the slide, their ft-ee ends being about 3 millimetres 



