34 THE FROG 



4. The opening into the bulbus cordis, guarded by three 

 pocket-like semi-lunar valves. 



5. The spiral valve attached to the wall of the bulbus 

 cordis. How does it end anteriorly and posteriorly? 



6. The truncus arteriosus. Cut one of its limbs across 

 not far from its origin. Observe that the cavity is divided 

 by two longitudinal partitions into three parallel tubes. Ob- 

 serve how these three tubes enter the main trunk; notice 

 especially the entrance of the pulmo-cutaneous artery (see 

 Fig. 72 of Holmes' Biology of the Frog). 



If possible, study the beating of the heart in a freshly- 

 killed frog. In what order do the various parts contract? 



C. The Arteries. These convey blood away from the 

 heart. A diagram (X^) of the arterial system should be 

 made showing all the arteries mentioned below. First ex- 

 amine a preparation or a model showing the principal ar- 

 teries; then study an injected specimen. With fine scissors 

 cut away the pericardium from the truncus arteriosus, then 

 with fine forceps carefully pick away the muscles and other 

 tissues wherever necessary to expose the arteries. If the 

 specimen is a female it may be necessary to remove the ovary 

 and oviduct from one side. If possible leave the specimen 

 in such condition that it may be used for the study of the 

 arteries by future classes. 



On each side, one of the two diverging arms of the trun- 

 cus arteriosus is continued into three arterial trunks or 

 aortic arches, which receive special names as follows : the 

 carotid arch, the systemic arch, and the pulmo-cutaneous 

 arch. 



I. The carotid arch, or common carotid artery, is the 

 most anterior of the three aortic arches. A short distance 



