788 DEVELOPMENT OP THE VASCULAR SYSTEM. 



especially in the human embryo, a blmit cleft or depression appears 

 betAveen this and the right part of the ventricle, which causes an 

 external division into two portions corresponding to the future right 

 and left ventricles ; and if the interior of the ventricular cavity be 

 examined at this time, there is perceived a crescentic partition rising 

 from the anterior or lower border of the right wall and projecting into 

 the cavity, at first narrow and placed opposite the external notch, but 

 gradually growing more and more towards the auriculo-ventricular 

 aperture. As development progresses the external division becomes more 

 or less effaced, when the apex of the heart formed by the left ventricle 

 becomes more pointed, and the whole heart takes more of the conical 

 form which belongs to its more advanced condition ; but the depression 

 is still perceptible as the interventricular groove of the adult heart, 

 which, as is well known, varies considerably in depth in different cases. 

 In some animals, as the rabbit, the temporary external division of the 

 ventricles is greater than in the human embryo, while in others, as in 

 ruminants, there is very little of the external notching, and in them, as 

 in birds, the heart very early assumes the conical form. The dugong 

 presents a remarkable example of the persistence of the complete ex- 

 ternal separation of the ventricles, and there appears to be a tendency 

 to the occasional occurrence of the same in the seal. 



The internal septum of the ventricles continuing to rise between the 

 right and left divisions of the cavity, reaches at last the base where it 

 is placed in relation with both the auriculo-ventricular orifice and the 

 root of the arterial bulb ; but at this place there remains for a time a 

 communication over the still free border of the septum between the 

 right and left ventricles, wdiich is interesting, as this is the seat of 

 the abnormal communication between the right and left ventricles in 

 almost all cases of malformation of the heart presenting that condition. 



Division of the Auricles. — Although the auricular cavity presents 

 externally some appearance of being divided into two at a period ante- 

 cedent to the partition of the ventricles, in consequence of the forma- 

 tion of the right and left auricular appendages before mentioned, the 

 internal division of the cavity does not take place till some time later, 

 as on the fifth and sixth days in the chick, and in the eighth week in 

 the human embryo. The auricular septum commences as an internal 

 fold proceeding from the anterior wall of the common cavity, and 

 starting from the septum of the ventricles, it grows backwards towards 

 the entrance of the common vein or sinus, but stops short of it 

 some distance. For a time, therefore, the veins enter the back part of 

 the auricular cavity in common. It is proper to explain, however, that, 

 by the time at which the auricular septum is forming, the venous 

 sinus has been modified so as to produce three veins entering the 

 auricle at its .back part. Of these, two correspond with the right 

 superior cava and the inferior cava veins, and the third to the left 

 superior cava and connected with what afterwards becomes the coronary 

 sinus. For a time, all the three vessels open so as to communicate 

 freely with the whole auricular cavity. But changes now occur which 

 cause the left superior cava and the inferior cava to be directed towards 

 the left side, Avhile the right superior cava is placed more immediately 

 in connection with the right part of the auricular cavity. 



The auricular septTim,in extending itself backwards, is not completed, 

 but leaves an oval deficiency in its lower and middle part, as the 



