22 THE FROG 
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 ventral 
surface; compare figure 72 of Holmes’ Biology of the Frog. 
Make a similar preparation for yourself by cutting away the 
ventral wall with scissors or a very sharp scalpel and exposing the 
following parts, all of which should be identified: 
1. The two auricles, right and left. Find the interauricular 
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. How do the valves function? 
2. On each side of the interauricular septum observe the open- 
ings of the auricle into the ventricle; these openings are guarded 
by valves. 
3. The cavity of the ventricle. Observe the spongy character 
of the walls, the spaces of which are continuous with the central 
cavity of the ventricle. 
4. The opening into the truncus arteriosus guarded by three 
pocket-like semi-lunar valves. 
5. The spiral valve attached to the wall of the truncus arteri- 
osus. How does it end anteriorly and posteriorly? 
6. The aortic arches. Cut one across not far from its origin. 
Observe that the cavity is divided by two longitudinal partitions 
into three parallel tubes. Observe how these three tubes enter the 
truncus; notice especially the entrance of the pulmo-cutaneous 
arch (see figure 73 of Holmes’ Biology of the Frog). 
If possible, study the beating of the heart in a freshly killed 
frog. In what order do the various parts contract? 
C. The Arteries.—These convey blood away from the heart. 
A diagram (X 2) of the arterial system should be made showing 
all the arteries mentioned below. First examine a preparation or 
a model showing the principal arteries; then study an injected 
