THE FROG 2a 
the diaphragm of the microscope very small. The white cor- 
puscles vary considerably in size, form and structure. The out- 
line is sometimes irregular and undergoes amceboid changes. 
The usually large granular nucleus may be either spherical or 
irregular in form; the cytoplasm may be either clear or granular. 
Often the cytoplasm is very granular so that the nucleus cannot 
be seen. Draw several white corpuscles differing in form or 
structure. If one is found actively changing in form, sketch it at 
intervals of about one-half minute, to show changes in form. 
B. The Heart.—The heart has the form of a cone with its 
apex directed posteriorly. The posterior part (ventricle) is 
thick-walled and opaque; the anterior part (auricles) is thin- 
walled and usually filled with clotted blood. Observe the large 
vessel, the truncus arteriosus, arising from the right anterior 
corner of the triangular ventricle; it passes anteriorly and to the 
left over the ventral surface of the auricles and branches im- 
mediately into the two aortic arches. Sketch a ventral view 
showing ventricle, auricles, truncus arteriosus and aortic arches; 
label all parts. Make this drawing small (X 2) and place it in 
the center of a clean page so that the arterial system may be 
added later. 
Gently lifting the apex of the heart, examine the dorsal surface 
carefully from behind and from the sides; notice the thin-walled 
sinus venosus, one of the divisions of the heart, filled with clotted 
blood, dorsal to the auricles. Three veins enter it: the posterior 
_vena cava from behind, the right and left anterior vene cave 
separately from in front. These veins, especially the posterior 
vena cava, are large, though like all the veins they have thin walls. 
The small pulmonary veins may also be seen arising from the 
inner faces of the lungs and uniting to form a single trunk, the 
short pulmonary vein proper, which enters the left auricle. 
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 sur- 
face of the sinus venosus. Notice the triangular form of the 
sinus and identify the three veins (venz cave) entering the three 
angles. If you are unable to recognize these parts, consult figure 
73 of Holmes’ Biology of the Frog. The pulmonary vein enters 
