THE FROG 33 



With scissors cut out the heart, being very careful to 

 leave the stumps of all the blood vessels attached, and place 

 in a small dish of water. Carefully clean bits of membrane, 

 etc., from the surface of the sinus venosus. Notice the 

 triangular form of the sinus and identify the three veins 

 (venae cavse) entering the three angles. If you are unable 

 to recognize these parts, consult Fig. 73 of Holmes' Biology 

 of the Frog. The pulmonary vein enters the left auricle just 

 in front of the anterior boundary of the sinus. Inserting 

 the point of your scissors into the posterior vena cava make 

 two diverging cuts running to the base of each anterior 

 vena cava. Turn back the flap thus formed, wash out the 

 clotted blood and examine the interior of the sinus. The 

 large transverse opening near the anterior end leads into 

 the right auricle. 



Examine a preparation of a heart dissected from the ven- 

 tral surface; compare Fig. 72 of Holmes' Biology of the 

 Frog. Make a similar preparation for yourself by cutting 

 away the ventral wall with scissors or a razor and exposing 

 the following parts, all of which should be identified : 



1. The two auricles, right and left. Find the interau- 

 ricular septum or thin membrane separating the two. 

 Which auricle is the larger? Find the opening of the sinus 

 venosus into the right auricle, and of the pulmonary vein 

 into the left; notice the valves at the opening of the sinus. 



2. On each side of the interauricular septum observe the 

 openings of the auricle into the ventricle; these openings are 

 guarded by valves. 



3. The cavity of the ventricle. Observe the spongy char- 

 acter of the walls, the spaces of which are continuous with 

 the central cavity of the ventricle. 



