PHYSIOLOGY OF CIRCULATION 275 



Tilt up the ventricle and observe the thin shred of connective 

 tissue the frenum (containing a small vein) -passing from the 

 pericardium to the back part of the ventricle. Tie a fine thread 

 around the frenum and divide it dorsal to the ligature. Lift the 

 heart gently by this thread and observe also the sinus venosus, 

 continuous with the right auricle and formed by the union of the 

 inferior and the two (smaller) superior venae cavae. 



2. Arrange to record heart beat on slow kymograph. Rate of 

 heart beat and influence of temperature. 



(a) Determine and record the normal rate per minute by 

 counting several times. 



(b) Determine and record the rate per minute after bathing 

 the heart by means of a pipette with a few c.c. of normal saline at 

 20 C.;at30 C. and 5 C. 



3. Movements and Sequence of Contraction. 



(a) Keep the heart beat slowed down with the cold normal 

 saline and it will be possible to determine the sequence and details 

 of contraction. Determine whether the two auricles contract at 

 the same time. * 



Carefully lift the ventricle by the ligature of the frenum and 

 observe the sinus venosus. Note where the wave of contraction 

 really begins and follow the whole cycle of contraction step by 

 step. Tabulate and describe the events in order through one 

 entire cycle. 



(b) Use a pithing needle now to destroy the entire spinal cord. 

 What effect do you observe upon the beating of the lymph hearts? 

 What can you conclude concerning the nerve centres which govern 

 their movements? Is any change produced in the beat or appear- 

 ance of the blood-heart? Conclusions? 



4. Effect of Temperature on the Excised Heart. Carefully 

 remove the entire heart from the body. 



(a) Place the watch glass containing the excised heart on a 

 small beaker of warm water determine the rate per second at 

 5 C., at 20 C. and at 35 C. Caution: Do not let the temperature 

 rise above 40 C. Plot beat curve and calculate Q 10. 



(b) Section of the excised heart. Separate the sinus, auricles 

 and ventricle from each other. Does each beat independently? Do 

 they beat at the same rates? Note time for 10 pulsations of each. 



