80 
LABORATORY EXERCISES 
tion. On it find a median, slit-like opening. This opening leads 
into the air passages which in turn open into the lungs. The back 
of the mouth leads into the esophagus. 
2. Examine a prepared dissection. Note the skin which has 
been cut and turned back. What do you observe with respect to 
its thickness and character? Beneath the skin is a muscular layer. 
When the body wall is opened and turned back, the contents of 
the cavity of the trunk are exposed. Note the thin membranes in 
whose folds some of the organs are suspended. These are the 
mesenteries. 
3. Find the heart at the anterior end of the body cavity. The 
pointed posterior part of it is the ventricle; the two dark-colored 
auricles are anterior and dorsal to this. From the right side of the 
base of the ventricle, a tube which forks into two branches carries 
the blood away from the heart. Each branch divides into three 
trunks, one pair of trunks uniting to form the dorsal aorta, the 
artery which supplies most of the body. The blood from the body 
returns by means of veins to the right auricle. On either side, 
near the heart, are the lungs. These connect with the opening in 
the mouth previously observed. The long digestive tube extends 
from the mouth to the anus. Beyond the esophagus lies the en¬ 
larged stomach. This connects with the coiled small intestine, 
which in turn leads into the large intestine. The three large, 
reddish-brown lobes of the liver lie at the side of and back of the 
heart. Situated in the loop formed by the intestine and stomach 
is the pancreas. On either side of the posterior part of the intes¬ 
tine in the dorsal region of the body cavity is a flattened reddish 
body, a kidney. Near the ventral surface of each kidney, in the 
male frog, is a white, elongated reproductive gland or testis. In 
the female, an ovary occupies a corresponding position. Each 
ovary is a large lobed mass, mainly composed of eggs. The eggs 
are black and white. The ducts of the ovaries are long and much 
coiled. They lie at either side of the ovaries. In both sexes a tuft 
of finger-like masses of yellow fat may be found connected with 
the reproductive glands. Find a thin sac, the urinary bladder, in 
the extreme posterior end of the body cavity. This is an outgrowth 
