316 Dr Allen Thomson on the Vascular System 



longer in its transverse than in its vertical diameter ; at the same 

 time, the canalis auricularis being farther shortened, the auricle is 

 more closelyapphed to the upper part of the ventricle. About the 

 80th hour, the commencement of the division of the auricle is in- 

 dicated externally by the appearance of a dark line on the upper 

 part of its wall, towards the left side, and very near the entrance 

 of the veins. In a few hours after this, we perceive that this dark 

 line (Fig. 18), is produced by a crescentic contraction which has 

 taken place on the upper and back part of the auricle, in a di- 

 rection at right angles to its longest diameter. This contraction 

 increasing, proceeds towards the ventricle, and soon divides the 

 long-shaped common auricle into two nearly spherical sacs ; of 

 these, the right (Fig. 18 r) is at first much the largest, and re- 

 ceives the superior and inferior venae cavae {x) ; the left small- 

 er, somewhat pointed at its free extremity, has at first the ap- 

 pearance of being a mere appendage of the right. Before the 

 end of the 6th day, this left auricle, at first quite free, becomes 

 connected with the pulmonary veins ; but the mode in which 

 this union is effected has not, so far as I know, been discover- 

 ed. The contraction in the wall of the auricle, proceeding from 

 above, continues to increase for more than a day after its first 

 appearance, so as at last to form a septum between the auricles ; 

 the foramen ovale is left towards the lower part of this septum, 

 so that the ventricle communicates freely with both cavities of 

 the auricle for some time. This foramen is afterwards closed 

 by the prolongation of a fold from behind forwards, in a di- 

 rection nearly opposite to that of the septum auricularum. 

 While the foramen ovale is yet quite open, there is seen at 

 the lower and back part of the right auricle a fold, appearing 

 to be a continuation of the septum, which is stretched across 

 the right auricle nearly horizontally from left to right. This fold 

 appears to correspond to the Tubercle of Lower and the Eusta- 

 chian valve (Fig. 21, B, v). It is situated between the mouths of 

 the upper and lower venae cavae, but does not appear, in the 

 examples in which I have examined it, to be adapted to direct- 

 in o- the flow of blood from the lower vena cava into the left auri- 

 cle. About the end of the 5th day, the auricles appear slightly 

 serrated on their edges, and, when emptied of blood, lose the 

 vesicular appearance which they had hitherto presented. The 

 anterior parts of both auricles now project more forward, and 



