22 THE FROG 



dium, dissect it clear from the heart on the ventral side. 

 Identify the following parts of the heart: 



94. The ventricle, a thick-walled muscular sac, forms 

 the apex of the triangular heart at its caudal end. 



95. The bulbus cordis on the ventral surface of the 

 heart arises from the ventricle on the right. It is 

 separated by a slight groove from the 



96. truncus arteriosus, a thinner walled tube assuming 

 the form of a Y at the cephalic end of the heart. 

 The two limbs of this Y lead to the aortic arches 

 (p. 26). 



97. Beneath the bulbus and truncus lie the atria, right 

 and left, which are thin walled, distensible sacs. 



Tip the heart forward and identify on the dorsal surface, 



98. the dorsal mesocardium, a membrane which is sus- 

 pended. Cut this to see 



99. the sinus venosus which receives the blood from 



100. the two large precaval veins from the cephalic side, 

 and 



101. the postcaval vein from the caudal end. 



102. The pulmonary vein, which brings blood from the 

 lungs, may be seen lying beside the left precaval vein. 

 It enters the left auricle. 



Drawing 9. Sketch the heart with its vessels (x2) 

 from both dorsal and ventral aspects. 



(The following dissection of the heart should be omitted 

 if R. catesbiana is not available.) 



Cut across the limbs of the truncus and the caval veins and 

 remove the heart, identifying again the structures you have 

 previously identified. 



To trace the course of the blood through the heart, place 

 it in a dish of water, dorsal side up. Carefully remove the 



