22 THE FROG 



Under the sternum, which can be raised, is a membranous 

 sac, the pericardium, containing the heart. Observe the 

 position of the pericardium with respect to the large dark- 

 colored lobed organ, the liver, filling the front part of the 

 body cavity. Observe the attachment of the pericardium 

 to the ventral body-wall by a vertical membrane which also 

 supports the liver. What relation does this membrane bear 

 to the anterior abdominal vein? Trace this vein forward 

 and find where it terminates anteriorly. 



B. The Viscera. 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 fol- 

 lowing 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 digestive tube is composed of the following parts: 



1 I ) The oesophagus, a short, wide tube, very distensible, 

 leads from the mouth cavity to the stomach. Find its open- 

 ing 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 an- 

 terior 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 in- 

 cision on its left side, wash out the contents and observe the 

 folds of the inner layer or mucosa. Examine the cut edge 



