EFFECT OF TEMPERATURES ON THE EXCISED HEART. 213 



(d.) Fix the tracings, and observe in the tracing a first 

 ascent due to the auricular contraction, and succeeding this 

 a second ascent due to the contraction of the ventricle, fol- 

 lowed by a slow subsidence due to the continuation of the 

 ventricular systole, and then a sudden descent due to the 

 diastolic relaxation of the heart. 



2. Auricular Contraction. Adjust the lever again so that it 

 rests on the auricles alone, and take a tracing. Note the smaller 

 excursion of the lever. In this case the cork resting on the 

 auricles must be small. 



3. Ventricular Contraction. Adjust the lever so as to obtain 

 a tracing of the ventricular movements only. 



4. In all the above experiments arrange an electro-magnetic 

 time-marker (Fig. 67) under the recording lever, so that the 

 points of the recording lever and time-marker write exactly in 

 the same vertical line with each other. In this way one can 

 calculate the time-relations of any part of the curve. The time- 

 marker is arranged to record seconds, and is driven by an electric 

 clock. 



5. Effect of Varying Temperatures on the Excised Heart. 



(a.) Excise the heart of a pithed frog, lay it on a cylindrical 

 brass cooling-box, three inches long and one broad, fixed to 

 a support, and fitted with an inlet and outlet tube, like 

 that in Fig. 62. Fix india-rubber tubes to the inlet and 

 outlet tubes of the cooling-box, the inlet tube passing from 

 a funnel fixed in a stand above the box, and the outlet tube 

 discharging into a vessel below it. Adjust the heart lever 

 to record the movements of the contracting ventricle on a 

 slowly revolving drum. If the heart tends to become dry, 

 moisten it with normal saline mixed with blood. Adjust a 

 time-marker, as indicated for other experiments. Take a 

 tracing. 



(6.) Pass water from 10 to 20 0. through the cooling-box, 

 noting the effect on the number of the contractions, and the 

 duration, height, and form of each single beat. 



