260 



PRACTICAL PHYSIOLOGY. 



[LII. 



during its diastole, it is distended with blood and has a red appear- 

 ance, the blood flowing into it being propelled by the contracting 

 auricles. Notice also how the position of the auriculo- ventricular 

 groove moves upwards and downwards during each cardiac cycle. 

 Note the normal rhythm, i.e., the number of beats per minute. 



4. Effect of Temperature (Heart in situ). 



(a.) By means of a pipette allow a few drops of normal saline at 

 2o-25 C. to bathe the heart, and note how rapidly the number 

 of beats, i.e., rhythm, is increased, and how each individual beat 

 is quicker. 



c.ad. 



A.*. 



FIG. 181. Heart of Frog from Behind. 

 s.v. Sinus venosus opened ; c.i. In- 

 ferior, c.s.d, c.s.8. Right and left 

 superior venae cavse ; v.p. Pulmonary 

 vein ; A.d, and A.s. Right and left 

 auricles ; A.p. Communication be- 

 tween the right and left auricle. 



FIO. 1 80. Frog's Heart 

 from the Front, v. Single 

 ventricle; Ad, As. Right 

 and left auricles ; B. 

 Bulbus arteriosiis ; i. 

 Carotid ; 2. Aorta ; 3. 

 Pulnio - cutaneous arte- 

 ries ; C. Carotid gland. 



(6.) Then apply normal saline at 10 C. or 5 C., and note the 

 opposite effect on the rate or rhythm, together with the slower 

 contraction of each individual beat. 



5. An Excised Heart Beats. 



(a.) With a seeker tilt up the apex of the ventricle, and observe 

 that a thin thread of connective tissue, called the " fraenum," 

 containing a small vein, passes from the pericardium to the posterior 

 aspect of the ventricle. Tie a fine silk thread round the fraenum 

 and divide it dorsal to the ligature. Count the number of beats 

 per minute. By means of the silk thread raise the heart as a 

 whole, and with a sharp pair of scissors cut out the heart by divid- 

 ing the inferior vena cava, the two superior venae cavae, and the 

 two aortas. Place the excised heart in a watch-glass, and cover it 

 with another watch-glass. 



(b.) The heart goes on beating. Count the number of beats per 

 minute. Therefore its beat is automatic, and the heart contains 

 within itself the mechanism for originating and keeping up its 

 rhythmical beats. 



