572 THE HUMAN EMBRYO. 



longitudinal ridge-like thickenings of the endothelial lining, 

 which, arising from opposite sides, encroach on the lumen, re- 

 ducing it to a slit, dumb-bell shaped in section; by further 

 growth, the two ridges meet each other and fuse, so as to divide 

 the lumen into two completely separate passages. 



The endothelial ridges, and consequently the septum itself, 

 appear first at the distal end of the truncus arteriosus, between the 

 origins of the systemic and pulmonary aortic arches, and gradu- 

 ally extend backwards towards the ventricle. The septum first 

 appears towards the end of the fourth week, and is completed 

 before the end of the fifth week ; it has a slightly spiral course, 

 so that the two tubes, into which it divides the truncus arteriosus, 

 are respectively dorsal and ventral at the proximal end, next to 

 the ventricle, and right and left at the distal end of the truncus. 



Of the two tubes, the one (Fig. 245, EX) which lies dorsally 

 at its proximal end, and on the right side distally, is the sys- 

 temic trunk ; the other, RW, which is ventral proximally, and on 

 the left side distally, is the pulmonary trunk ; and the same 

 relations are retained throughout life by the ascending aorta 

 and the root of the pulmonary artery, into which the trunks 

 develop respectively. 



The separation of the systemic and pulmonary trunks at first 

 concerns their internal cavities alone ; but it is soon followed by 

 the appearance of external grooves, which deepen until they 

 completely separate the two trunks from each other. 



The interventricular septum. The truncus arteriosus origi- 

 nally arises from the right-hand corner of the ventricular cavity, 

 and the two trunks into which it becomes split retain for a 

 time the same relations. In other words, at a time when the 

 interventricular septum is already partially formed (Fig. 244, 

 SD), both the systemic and pulmonary trunks arise from the 

 right ventricle, and the left ventricle has for a time no outlet, 

 except through the right ventricle. 



The completion of the interventricular septum has to be 

 effected in such a way that while the pulmonary trunk is left in 

 connection with the right ventricle, the' systemic trunk shall be 

 cut off from the right ventricle and placed in communication 

 with the left ventricle. 



The formation of the interventricular septum is consequently 

 somewhat complicated. The greater part of the septum is 



