HEART-BEAT OF FROG 



63 



d. The pause, or diastole, which follows before the 

 auricles again beat; if the heart is beating rapidly this 

 may not be obvious to the eye. 



e. The increased redness and distension of the ven- 

 tricle after the auricular, arid immediately preceding its 

 own systole. 



2. Tilt forward the apex of the ventricle, a thin 

 thread of connective tissue the ligament of the ven- 

 tricle will be seen passing from its dorsal surface to 

 the pericardium; cut this through. Pull the ventricle 

 forward by the ligament. Observe 



a. The junction of the two superior venae cavse with 

 the inferior vena cava to form the sinus venosus (cp. 

 Fig. 19). 



b. The whitish line mark- 

 ing the junction of the 

 sinus venosus with the 

 right auricle. 



c. The wave of con- 

 traction; it starts in the 

 endings of the great veins 

 (the progress of the wave 

 may be seen in the su- 

 perior venae cavse); then 

 follow in quick succession, 

 contraction of the auricles, 

 the ventricle, and the bul- 

 bus arteriosus. 



Fig. 19. Diagram of heart of frog 

 with the ventricle turned forwards. 



3. Dissection of the vagus. Make a transverse cut 

 through the skin of the frog just below the lower jaw, 

 and carry the cut as far as the vertebral column. Cut 



