VII.] THE BULBUS ARTERIOSUS. 193 



directed nearly backwards (i.e. towards the tail), but also a 

 little downwards. 



An alteration takes place during the sixth day in the 

 relative position of the parts of the ventricular division of 

 the heart. The right ventricle is now turned towards the 

 abdominal surface, and also winds to a certain extent round 

 the left ventricle. It will be remembered that on the fourth 

 day the right ventricle was placed above (dorsal to) the left. 



The right ventricle is now also the smaller of the two, 

 and the constriction which divides it from the left ventricle 

 does not extend to the apex of the heart (Fig. 60) It has, 

 however, a very marked bulge towards the right. 



At first the bulbus arteriosus appeared to come off 

 chiefly from the left ventricle ; during the fifth day, and 

 still more on the sixth, it appears to come from the right 

 chamber. This is caused by the canal from the right ven- 

 tricle into the bulbus arteriosus passing towards the left, 

 and on the ventral side, so as entirely to conceal the origin of 

 the canal from the left chamber of the heart. On the seventh 

 day the bulbus arteriosus appears to come less markedly from 

 the right side of the heart. 



All these changes, however, of position of the bulbus 

 arteriosus only affect it externally; during the whole time 

 the two chambers of the heart open respectively into the two 

 divisions of the bulbus arteriosus. The swelling of the 



FIG. 61. 

 T.O, 



T.V 



HEART OF A CHICK UPON THE SIXTH DAY OF INCUBATION, FBOM THE 

 VENTRAL SURFACE. 



I. a. left auricular appendage, r.a. right auricular appendage, r.v. right ventricle, 

 I. v. left ventricle, b. bulbus arteriosus. 



E. 13 



