144 DEVELOPMENT OF THE HEART. 



with the fused cushion-like thickenings which are subdividing' the' common auricular 

 orifice. But before the originally free communication between the two auricles is thus 

 closed, a new aperture makes its appearance above and at the back of this septum, 

 and gradually enlarges, so that a passage is thus re-established, but in a different 

 situation. This new orifice is the foramen ovale, it becomes closed by a second 

 septum, which also starts from the superior auricular wall, a little to the right of the 

 original attachment of the first septum, and gradually grows forwards and downwards 

 over the orifice. This second septum becomes the limbus Vieussenii, the first one 

 forms the so-called valve of the foramen ovale (Born). 



According to His' account of the process in the human embryo, the septum atriorum is 

 formed by an anterior, or lower, and a posterior, or upper, sickle-shaped projection, which between 

 them enclose the foramen ovale, and form respectively the limbus Vieussenii and the valve of 

 the foramen ovale ; the connective tissue growth which he describes as growing from the 

 posterior auricular wall towards the auric ulo- ventricular orifice takes an important part in 

 the formation of the lower septal projection (septum intermedium). There is reason, how- 

 ever, to believe that the process, as above described by Born for the rabbit, is materially the 

 same in all the higher vertebrates, including man, and that the successive growth of both 

 septa from the upper and posterior auricular wall was not noticed by His on account of the 

 lack of a series of human embryos sufficiently complete to show all the stages of growth. 



Somewhat late in the course of development (after the appearance of the auricular 

 septum), the pulmonary veins are seen entering the left auricle. Before reaching the 

 auricle they have united to form a single vessel, and this opens into the auricle near 

 the septum (fig. 170, p.v.). In some animals, as the rabbit, this represents the 

 permanent mode of termination of the pulmonary veins, but in man the right and 

 left veins come to open separately into the auricular cavity, either by division of the 

 common trunk (His), or by opening out of the common trunk, and its absorption 

 into the auricle in the same way as the right horn of the venous sinus is absorbed 

 into the right auricle (Born). The two resulting vessels may again divide, so that 

 four pulmonary veins ultimately terminate in the left auricle. 



The aortic lulb becomes subdivided into two vessels, the ascending aorta and 

 the pulmonary artery. The division is produced by a septum which arises as two 

 longitudinal thickenings of the lining membrane (endocardium). These grow from 

 opposite sides, and gradually meeting, fuse together in the middle of the bulb. The 

 folds take an oblique course down the bulb, for above they are anterior and posterior, 

 but below are right and left, hence the resulting vessels after separation are anterior 

 and posterior below and right and left above. The endocardial thickenings extend 

 somewhat below the origin of the bulb, and unite with one another and with the 

 septum of the ventricles, which they complete, and of which they form the mem- 

 branous part. The ventricular part of the heart is now completely divided into two, 

 each communicating with the corresponding division of the arterial bulb. There are 

 at first no semilunar valves, the soft thickened endocardial tissue of the bulb appear- 

 ing to exercise a sort of valvular action. The valves are formed as three projec- 

 tions of this tissue at the base of each vessel, at first thick and soft, but subsequently 

 becoming thinner and membranous. The common aortic trunk has four such 

 thickenings at the lower end, and the septum of the bulb as it descends is prolonged 

 into the right and left of these, so that the dumb-bell-shaped orifice is divided into 

 two triangular apertures, the bulging sides of which are formed by the endocardial 

 cushions. and become developed into the semilunar valves (fig. 174). 



The aortic septum begins between the fourth and fifth aortic arches, and is so dis- 

 posed, that the fourth arch continues the aortic half of the bulb, the fifth the pul- 

 monary half. After the completion of the seplum, an external groove makes its 

 appearance along the line of the endocardial thickenings, and deepening gradually, 

 splits the bulb into two separate vessels. 



