THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 559 



from above downward until it reaches the middle of the atrial canal 

 (fig. 309 si). In this manner two completely separated atria would 

 have come into existence at a very early period, if there had not 

 been formed in the upper part of the partition, while it was still 

 growing downward, an opening, the future foramen ovale, which 

 maintains a connection between the two chambers (fig. 309) up to 

 the time of birth. The opening has arisen either from the septum 

 atriorum having become thin and having broken through at a 

 certain region, or from its having been incomplete at this place 

 from the very beginning, as is the case with the Chick for example, 

 where it is traversed 

 by numerous small 

 orifices. Afterwards 

 the foramen ovale, 

 adapting itself to 

 the conditions of the 

 circulation existing 

 at the time, becomes 

 still larger. 



The downgrowth 

 of the atrial parti- 

 tion has, moreover, 

 the immediate result 

 of separating the au- 

 ricular canal into the 

 left and right atrio- 

 ventricular orifices 

 (compare fig. 308 ok 

 with fig. 309). The 

 auricular canal, even 



very soon after its formation, undergoes important alterations 

 both from without and within. At first visible from the out- 

 side (fig. 308 ok), it afterwards disappears from view (fig. 309) 

 by being in a manner overgrown on all sides by the ventricle, 

 and thereby incorporated in its walls, which enlarge upward and, 

 in consequence of a vigorous growth of the musculature, acquire con- 

 siderable thickness. The opening of the atrial canal into the ven- 

 tricle, or the foramen atrioventriculare commune (fig. 310 A F.av.c), 

 now has the form of a fissure extending from left to right, which 

 is bounded on either side by two ridge-like lips (o.ek and u.ek) 

 the atrioventricular lips of LIN DBS, or the endothelial cushions of 



Fig. 309. Posterior [dorsal] half of the heart of a human 

 embryo of the fifth week, cut open, after His. 



ks, Ventricular partition ; Ik, left, rk, right ventricle ; si, lower 

 [posterior] part of the atrial partition (septum intermedium, 

 His) ; Iv, left, vv, right atrium ; sr, mouth of the sinus 

 reunions ; vs, atrial partition (atrial crescent, His ; septum 

 secundum, BORN) ; Ps, septum spurium ; * Eustachian 

 valve. 



