xiii.] THE FROG. 177 



order in which they have just been named. Each contracts 

 as a whole, so that the two auricles are emptied simulta- 

 neously. The blood from each is forced into the correspond- 

 ing half of the spongy cavity of the ventricle, so that the 

 right half of the ventricle contains venous blood and the 

 left arterial blood. When the systole of the ventricle takes 

 place, the blood which is first driven into the truncus arte- 

 riosus (the opening of which is, as has been seen, at the 

 right end of the cavity) is therefore venous. It fills the 

 conus arteriosus and, finding least resistance in the short 

 and wide pulmonary vessels, passes along the left side of the 

 median valve into them. But as they become distended 

 and less resistance is offered elsewhere, the next portion 

 of blood, consisting of the venous and arterial blood which 

 have mixed in the middle of the ventricle, passes on the 

 right side of the longitudinal valve into the aortic arches. 

 And, as the truncus becomes more and more distended, the 

 longitudinal valve, flapping over, tends more and more 

 completely to shut off the openings of the pulmonary 

 arteries and to prevent any blood from flowing into them. 



Finally, the last portion of blood from the ventricle, 

 representing the completely arterialized blood of the left 

 auricle which is the last to arrive at the opening of the 

 truncus, passes into the carotid trunks and is distributed to 

 the head. 



The principal vessels of the Frog are disposed as fol- 

 lows : 



A. Arteries. 



i. The system of the anterior aortic arch (carotid 

 trunk). 



a. Lingual artery to the tongue. 

 M. 12 



