DEVELOPMENT OF THE VEINS. 1035 



of the dorsal part of the septum primum which is not yet fused with the septum secundum 

 acts as a flap valve, permitting blood to pass from the right to the left atrium, but preventing 

 its return. This condition persists until birth, when the thickened free margin of the septum 

 secundum fuses with the right lateral face of the septum primum, and the foramen ovale of the 

 foetus becomes the fossa ovalis of the child. The fossa ovalis is bounded ventrally and caudally 

 (anteriorly and inferiorly) by the limbus fossae ovalis, which is formed mainly by the originally 

 free margin of the septum secundum, but to a small extent also by the angle between the right 

 cornu and the intermediate part of the sinus venosus. 



The Division of the Primitive Ventricle, the Bulbus Cordis, and the Truncus 

 Arteriosus. Two facts have already been pointed out with regard to the bulbus cordis ; firstly, 

 that it forms part of the cranial or anterior limb of the early loop-shaped heart, and secondly, 

 that it disappears by being absorbed partly into the ventricle and partly into the truncus 

 arteriosus. The part absorbed into the ventricle enters into the formation of that part of the 

 ventricle which afterwards becomes the right ventricle of the completed heart, and the part 

 absorbed into the truncus arteriosus lengthens that trunk. 



The division of the elongated truncus arteriosus into the pulmonary artery and the ascending 

 aorta is a complicated process in which three factors are associated : (1) a proximal and (2) a 

 distal set of eiidocardial swellings in the bulbar part of the truncus aorticus, which are known as 

 the proximal and distal bulbar swellings, and (3) a septum called the aorto-pulmonary septum, 

 which appears at the cephalic end of the truncus aorticus, growing from the angle between the 

 orifices of the more dorsally situated sixth aortic arches, and the orifices of the two stems which 

 form the ventral roots of the first four aortic arches. The three sets of elements are, at first, 

 entirely distinct from one another, but ultimately they blend together to form a spiral septum 

 by which the lumen of the truncus arteriosus is divided into two channels. One of the two 

 channels communicates with the right ventricle and the other with the left ventricle. After 

 the septum is completed it is cleft longitudinally into two parts, and so the truncus is divided 

 into two vessels, the pulmonary artery which communicates with the right ventricle, and the 

 aorta which is connected with the left ventricle. 



The proximal bulbar swellings, which take part in the separation of the truncus into 

 pulmonary artery and aorta, are. prolonged into the ventricular region as the ventral part of the 

 bulbus cordis is absorbed into the ventricle, consequently the septum which they form by their 

 fusion is prolonged into the ventricular chamber, and helps to separate that cavity into two parts, 

 by fusing with the dorsal end of the cephalic or anterior portion of the interventricular septum. 



The interventricular septum appears as a semilunar ridge on the inner surface of the ventral 

 part of the wall of the ventricle. At a later period its position is marked on the outer surface of 

 the ventricle by the interventricular sulcus (Fig. 829) which persists in the completed heart. As 

 the interventricular septum increases in height it fuses with the fused endocardial cushions which 

 divided the atrio-ventricular canal into right and left parts, and with the ventral border of the 

 bulbar septum which projects into the dorsal part of the ventricular chamber. The completed 

 interventricular septum consists, therefore, of two distinct parts : a ventral part formed by the 

 proper interventricular septum, and a dorsal part formed by the lower portion of the fused bulbar 

 endocardial cushions. The twojparts can be distinguished quite easily in the adult heart, for the 

 interventricular septum is muscular, and it forms by far the greater part of the permanent 

 septum, whilst the bulbar part of the septum, being developed from endocardial thickenings, is 

 membranous, and it forms the small pars membranacea of the permanent septum, which lies 

 between the aortic vestibule of the left ventricle and the dorsal part of the right ventricle and 

 the adjacent ventral part of the right atrium. 



DEVELOPMENT OF THE VEINS. 



Simultaneously with the formation of the arteries, by which blood is distributed to the 

 embryo and to all parts of the zygote, and in a similar manner, a series of vessels is developed 

 by means of which the blood is returned to the heart. These vessels are the primitive veins ; 

 they form two main groups which make their appearance at different periods. 



The first group consists of (1) the vitelline veins which return blood from the yolk sac, and 

 (2) the umbilical veins which return the blood from the placenta. 



The second group consists of (1) the anterior cardinal veins, (2) the posterior cardinal veins, 

 the (3) ducts of Cuvier, and (4) the subcardinal veins. 



The ducts of Cuvier, one on each side, are the common stems by which the blood of the 

 anterior and posterior cardinal veins is conveyed to the primitive heart. The anterior cardinal 

 vein returns blood from the head and neck and fore limbs. The posterior cardinal veins carry 

 blood from the body walls, the Wolffian bodies, and the hind limbs. The subcardinal veins also 

 are closely associated with the return of blood from the Wolffian bodies. 



The veins of the first group are largely transitory. The umbilical veins entirely disappear 

 as blood channels, but a part of the left lateral umbilical vein is recognisable in the adult as the 

 ligamentum teres of the liver. Portions of the intra-embryonic parts of the vitelline veins, 

 and anastomoses which form between them, remain as the portal vein, its right and left 

 branches, and the upper end of the inferior vena cava. 



The anterior cardinal veins, their tributaries, and a transverse anastomosis which forms 

 between them in the more cranialward or upper portion of the thoracic region, are converted 

 into the (1) cavernous sinuses of the cranium, (2) the internal jugular veins, (3) the innominate 

 veins, (4) the upper part of the superior vena cava, and (5) the upper part of the left superior 



