14 THE FROG 
Trace this vein backward and forward until it leaves the body 
wall, and find where it terminates anteriorly. 
After carefully freeing the sternum from the pericardium, pin 
back the flaps of the body wall so as to expose the internal organs 
(viscera). Find the following organs: . 
1. The heart, already noticed. In a freshly killed frog the 
heart usually continues to pulsate for a considerable time. The 
large veins entering the heart will be found dark-colored because 
filled with blood; the arteries are white because after death they 
contain little blood. 
2. The digestive system. (a) The alimentary canal or diges- 
tive tube is composed of the following parts: 
(1) The esophagus, a short, wide tube, very distensible, leads 
from the mouth cavity to the stomach. Find its opening into the 
mouth cavity, and insert a probe through the mouth cavity into 
the stomach. 
(2) The stomach, a whitish organ, wider and with thicker walls 
than the rest of the alimentary canal. Its anterior end is known 
as the cardiac end and its posterior end as the pyloric end. 
Observe the pyloric constriction; what is its function? Cut open 
the stomach by a longitudinal incision on its left side, wash out 
the contents and observe the folds of the inner layer or mucosa. 
Examine the cut edge with a lens and note just outside of the 
mucosa a spongy layer, the sub-mucosa, which is surrounded by a 
third layer, the muscular layer, and this in turn is covered ex- 
ternally by the very thin glistening peritoneum. 
(3) The small intestine, into which the stomach is continued at 
its pyloric end. The first portion, which runs anteriorly, is the 
duodenum. Under cover of the liver the duodenum bends pos- 
teriorly and the part of the small intestine posteriur to the bend is 
known as the ileum. The latter, after a number of coils, passes 
abruptly into: 
(4) The large intestine. The posterior part of the large in- 
testine is called the cloaca; this opens to the exterior by the anus. 
If the animal has not been too long dead, observe the peristaltic 
movements of the intestine (pinching the small intestine with fine 
forceps will usually start these movements). What is the func- 
tion of these peristaltic movements? The entire alimentary canal 
