DEVELOPMENT OF THE HEART. 



88 3 



structure elongates into a tube, which very soon assumes a shape somewhat like an S (fig. 675,1)], 

 and there are indications of its being subdivided into (a) an upper aortic part with the bulbus 

 arteriosus ; (b) a middle or ventricular part ; and (1;), a lower venous or auricular part. The 

 heart then curves on itself in the form of a horse-shoe (2), so that the venous end (A) comes to 

 lie above and slightly behind the arterial end. On the right and left side, respectively, of the 

 venous part is a blind hollow outgrowth, which forms the large auricle on each side (3, 0, Oj). 

 The flexure of the body of the heart corresponding to the great curvature (2, V) is divided into 

 two large compartments (3), the division being indicated by a slight depression on the surface. 

 The large truncus venosus (4, v), which joins with the middle of the posterior wall of the 

 auricular part, is composed of the superior and inferior vense cava?. This common trunk is 

 absorbed at a later period into the enlarging auricle, and thus arise the separate terminations 

 of the superior and inferior vense cava?. In man, the heart soon comes to lie in a special cavity, 

 which in part is bounded by a portion of the diaphragm {His). At the 4th to 5th week, the heart 

 begins to be divided into a right and a left half. Corresponding to the position of the vertical 

 ventricular furrow, a septum grows upwards vertically in the interior of the heart, and divides 



Fig. 675. 



Development of the heart. 1, Early appearance of the heart a, aortic part, with the bulbus, 

 b ; v, venous end. 2, Horse-shoe shaped curving of the heart a, aortic end, with the 

 bulbus, b ; V, ventricle ; A, auricular part. 3, Formation of the auricular appendages, 

 0, o v and the external furrow in the ventricle. 4, Commencing division of the aorta, 

 p, into two tubes, a. 5, View from behind of the opened auricle, v, v, into the L, and 

 R, ventricles, and between the two latter the projecting ventricular septum, while the aorta 

 {a) and pulmonary artery {p) open into their respective ventricles. 6, Relation of the 

 orifices of the superior {Cs) and inferior vena cava {Gi) to the auricle (schematic view from 

 above) x, direction of the blood of the superior vena cava into the right auricle ; y,, that of 

 the inferior cava to the left auricle ; tL, tubercle of Lower. 7, Heart of the ripe foetus R, 

 right, L, left ventricle ; a, aorta, with the innominate, c, c, carotid, c, and left subclavian 

 artery, s ; B, ductus arteriosus ; p, pulmonary artery, with the small branches 1 and 2, to 

 the lungs. 



the ventricular part into a right and left ventricle (5, R, L). There is a constriction in the 

 heart, between the auricular and ventricular portions, forming the canalis auricularis. It 

 contains a communication between the auricle and both ventricles, lying between an anterior 

 and posterior projecting lip of endothelium, from which the auriculo-ventricular valves are 

 formed {F. Schmidt). The ventricular septum grows upwards towards the canalis auricularis, 

 and is complete at the 8th week. Thus, the large undivided auricle communicates by a right 

 and left auriculo-ventricular opening with the corresponding ventricle (5). At the same time 

 two septa (4, p a) appear in the interior of the truncus arteriosus (4, p), which ultimately meet, 

 and thus divide this tube into two tubes (5, ap), the latter forming the aorta and pulmonary 



