142 DEVELOPMENT OF THE HEART. 



orifices ; (2) from the endo-cardial aortic septum, which divides the bulb into aorta 

 and pulmonary artery. Thus the septum of the ventricles is completed by endo- 

 cardial connective tissue, a fact which is indicated even in the adult heart by the 

 existence of the thin septum membranaceum which forms the uppermost part of 

 the inter- ventricular septum. 



The common auricle in the meantime becomes shifted relatively upwards over 

 the back of the ventricles, carrying the sinus along with it, but it still lies behind 

 rather than over the ventricles, and the aperture of communication passes from 

 behind forwards, from the left part of the auricle into the corresponding half of the 

 ventricle. This constricted aperture soon becomes elongated into a short canal, 

 which is known as the auricular canal. Its orifice into the ventricle is from the 

 first somewhat flattened, and bounded by two lips, an upper and a lower. As deve- 

 lopment proceeds, it broadens out towards the median plane of the ventricular tube, 

 and becomes gradually shifted, first towards, and eventually over the line of constric- 

 tion which, marks off the future right and left ventricles from one another (fig. 172). 

 The ventricular septum has by this time extended almost, up to the transversely 

 elongated slit-like orifice, and its lips, still upper and lower in relative position, 

 become greatly thickened by the formation of cushions of endocardium, which grow 

 towards one another in the middle of the slit, and presently fuse into a median 

 thickening which converts the single M-shaped aperture into two triangular 

 openings, leading one into each ventricle (fig. 172, 0). Meanwhile, the septum of 

 the ventricles growing towards the base abuts against, and at length comes into direct 

 continuity with the fused endocardial cushions, but this connection is nearer to the 

 right than to the left auriculo-ventricular aperture. 2 There is still, as above stated, 

 a small orifice of direct communication between the left and right ventricles above 

 the free edge of the ventricular septum, and this is not closed until the descent of the 

 septum of the bulb, and its union with the septum of the ventricles, completes the 

 interventricular septum. 



The above account of the division of the auricular canal is based upon that given 

 by Born for the rabbit, and in some respects differs from the description which was 

 given by His from an examination of human embryoes. According to His, the endo- 

 cardial cushions, which by their union subdivide the auricular canal, are preceded by 

 and connected with a growth of endocardial tissue, which springs from the posterior 

 auricular wall, and they together form a septal prolongation (septum intermedium), 

 which projects like a stopper into the auricular canal, and divides the latter into the 

 two auriculo-ventricular orifices, and also grows down beyond that canal to meet the 

 uprising ventricular septum (fig. 173). The shortening of this canal is in part effected 

 by a kind of intussusception which takes place, and which causes its wall to be 

 folded into the ventricular cavity ; these folds, with probably some thickening of 

 endocardium, form the bases of the lateral flaps of the auriculo-ventricular valves 

 (fig. 175). The bases of the mesial or septal flaps are formed by a downward growth 

 of the edges of the endocardial septum between the two orifices. Both lateral and 

 mesial flaps become continuous with the spongy muscular substance which at this 

 time occupies most of the cavity of the ventricles (fig. 175). As development 

 proceeds, the flaps, which are at first thick and soft, become thin and membranous, 

 and become free from muscular substance except near their free edges. These 

 muscular bands become tendinous near their insertion into the valves, and thus form 

 the chordce tendinece ; the parts which are not thus transformed become the papillary 

 muscles. 



The septum of the auricles appears at the upper and back part of the auricular 



2 Hence the right auriculo-ventricular orifice lies close to the ventricular septum, but the left orifice 

 is separated from it by an interval, into which the root of the aorta becomes continued (Born). 



