DIVISION OF THE AUKICLES. 



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foramen ovale, and the inferior cava opens immediately behind this. 

 Some time later in the human embryo, or in the course of the tenth or 

 eleventh weeks, two new folds make their appearance in the auricles 

 posteriorly. One of these constituting the Eustachian valve, of a 



Fig. 588. — Shows the position and 



FORM OP THE HeART IN THE HuMAN 



Embryo from the Fourth to the Sixth 



WEEK. 



A, upper half of tlie body of a human 

 embryo of Dearly four weeks old (from 

 Kolliker after Coste) ; B and C, anterior 

 and posterior views of the heart of a human 

 embryo of six weeks (from Kolliker after 

 Ecker) ; a, frontal lappet ; h, mouth ; c, 

 below the lower jaw and in front of the 

 second and third branchial arches ; d, 

 upijer limb ; e, liver ; /, intestine cut 

 short ; 1, superior vena cava ; 1', left 

 superior cava or brachio-cephalic connected 

 with the coronary vein ; 1", opening of the 

 inferior vena cava ; 2, 2', right and left 

 auricles ; 3, 3', right and left ventricles ; 

 4, aortic bulb. 



crescentic form, is placed to the right of the entrance of the inferior 

 vena cava, and in the angle between it and the orifice of the left supe- 

 rior cava (or great coronary sinus), and besides separating these two 

 veins, and thus throwing the opening of the left superior cava into 

 communication with the right auricle, this fold, as it runs forward into 

 the annulus ovalis or border of the anterior auricular septum, has the 

 effect of deepening the entrance of the inferior cava into a groove 

 which lies close to the foramen ovale, and directs the blood entering by 

 that vessel through the passage into the left auricle. 



The other fold referred to advances from the posterior wall of the 

 common auricle to meet the anterior auricular septum, but yet to the left 

 of the border of the foramen ovale. To this border, however, it adheres 

 as it grows forwards, and thus gradually fills up the floor of the fossa 

 ovalis. Up to the middle of foetal life, this posterior septum being 

 incomplete, there is a direct passage from right to left through the 

 foramen ; but, after that period, the fold in question, having advanced 

 beyond the anterior border of the annulus ovalis and lying to the left, 

 it does not adhere to this or the fore part of the annulus, but leaves a 

 passage between, and appears as a crescentic fold in the left auricle, 

 which, as it passes beyond the annulus, constitutes a very perfect valve 

 against the return of blood from the left into the right auricle. 



Division of the Arterial Bulb. — The third important change 

 occurring in the heart belongs to the arterial bulb, by which there are 

 developed from this tube the first parts or main stems of the pulmonary 

 artery and the ao'-ta. Within the thick walls of this arterial tube 

 there is at first only a single cylindrical cavity, continued from the 

 originally single ventricle ; but, a short time after the partition of the 

 ventricular cavity has commenced, or in the seventh week of the human 

 embryo, a division of the bulb by an independent process begins to 



