266 THE ANATOMY OF VEKTEBRATED ANIMALS. 



cross one another as they do so, because the origin of the left 

 arch lies more to the right than does the origin of the right 

 arch. The ostia of both arches are guarded by semi-lunar 

 valves ; and that of the left arch is placed below and to the 

 right of that of the right arch. As no arterial trunk arises 

 from the cawm m'terlosutn, the red blood can be driven out 

 of the latter, during the systole, only into the cavum venosum. 



The right, comparatively small, moiety of the ventricle 

 is separated from the cavum venosum by the already-men- 

 tioned septum, which is attached between the origin of the 

 left aortic arch and that of the pulmonary artery, its free edge 

 looking toward the dorsal face of the heart. Thus the pul- 

 monary arterj' arises from what is, virtually, a separate sub- 

 division of the ventricle, or a cavum pulmonale. 



When the systole of the ventricle takes place, the practical 

 result of these arrangements is, that the pulmonary artery, 

 and the aortic arches, at first, receive wholly venous blood 

 from the cavum, venosum and cavum pulmonale. But as the 

 arterial blood of the cavum, arteriosum is driven into the 

 cavum venosMm, the venous blood of the latter tends to be 

 excluded from the mouths of the aortic arches, and to be 

 driven into the cavum pulmonale, while the aortic arches 

 receive arterialized blood. The left arch receives a larger pro- 

 portion of venous blood than the right. As the ventricle 

 contracts, the free edge of the muscular septum approaches 

 the dorsal wall of the ventricle, and gradually closes the access 

 to the cavum pulmonale, which thus finally expels the venous 

 blood which it received from the cavum venosum,, but admits 

 none of the arterialized blood ; consequently none of this 

 reaches the lungs. 



2. In the Crocodilia, the cavum, venosum and the cavum, 

 arteriosmn are converted into perfectly distinct right and left 

 ventricles. The right ventricle gives off the pulmonary artery, 

 and, in addition, an aortic arch ^\-hich crosses over to the left 

 side. From the left ventricle only a single trunk arises, and 

 this, crossing to the right side, becomes the right aortic arch, 

 of which the dorsal aorta is the direct continuation. The walls 

 of the two aortic arches are in contact where they cross one 

 another ; and, at this point, a small aperture, situated above 

 the semilunar valves, places the cavities of the two arches in 

 communication. 



Thus, in the Crocodilia, the venous and the arterial cur- 

 rents communicate only outside the heart, not within it as In 

 the foregoing groups. 



