V.] THE HEART. 123 



Starting from the point of junction of the omphalo-mesaraic 

 veins (Fig. 24, Ht), there is first a slight curvature towards the 

 left ; this is followed by a turn to the right, and then the heart 

 is completely bent on itself, so that afterwards it pursues a 

 course directed from behind quite straight forwards (except 

 perhaps for a little inclination to the left) to the point where 

 the aortic arches branch off. In this way the end of the 

 bulbus arteriosus comes to lie just underneath (or in front of 

 according to the position of the embryo) that part which has 

 already been marked off by the lateral bulgings as the 

 auricular portion. 



That part of the heart which is turned to the right, 

 including the point of doubling up, is the ventricular portion, 

 and is even at this stage separated from the auricular portion 

 by a slight neck. This external constriction corresponds to 

 an internal narrowing of the lumen of the heart, and marks 

 the position of the future canalis auricularis. 



The ventricular portion is, on the other hand, likewise 

 separated by a fainter constriction from the anterior continua- 

 tion of the heart which forms the bulbus arteriosus. The 

 projecting part where the doubling takes place is at this 

 stage still quite round ; we shall see that later on it becomes 

 pointed and forms the apex of the heart. 



The whole venous portion of the heart (if we may so 

 speak, though of course at this stage blood of the same 

 quality passes right along the whole cardiac canal) lies in a 

 plane which is more dorsal than the arterial portion. The 

 point at which the venous roots of the heart, i. e. the two 

 omphalo-mesaraic trunks unite into a single canal, is on this 

 day carried farther and farther away from the heart itself. 

 By the end of the day there is a considerable distance 

 between the auricular portion of the actual heart and the 

 point where the venous roots separate each to pursue its 

 course along the splanchnopleure-fold of its own side. This 

 distance is traversed by a single venous trunk, of which the 

 portion close to the auricles is called the sinus venosus, and 

 the more distant the ductus venosus. We shall give to the 

 whole trunk the name used by the older observers, the 

 meatus venosus. 



Small arteries to various parts of the body are now 

 being given off by the aorta and its branches. The capil- 



