LATER DEVELOPMENT OF VASCULAR SYSTEM 351 



appropriate valves are formed. We have thus to describe the 

 origin of three primary septa, viz., the interauricular septum, 

 the interventricular septum, and the septum of the truncus and 

 bulbus arteriosus. These do not, however, themselves unite 

 directly, but are joined together by the intermediation of a fourth, 

 large, cushion-like septum formed in the auricular canal, i.e., in 

 the opening between the primitive atrium and ventricle. 



In general it may be said that the development of the three 

 primary septa takes place from the periphery towards the center, 

 i.e., towards the cushion-septum of the auricular canal, and that 

 it is practically synchronous in all three, though there is a slight 

 precedence of the interauricular septum. During the same time 

 the cushion-septum of the auricular canal is formed. We may 

 then consider first the origin of these septa separately, and second 

 their union. 



(a) The Septum Trunci et Bidbi Arteriosi (Septum Aortico- 

 Pulmonale). This septum divides the truncus and bulbus arte- 

 riosus into two arteries, the aorta and pulmonary artery. Three 

 divisions may be distinguished, viz., a part in the truncus arte- 

 riosus, a part in the distal division of the bulbus extending to 

 the place of formation of the semilunar valves, and a part in the 

 proximal portion of the bulbus, which subsequently becomes 

 incorporated in the ventricles. In mode of formation these are 

 more or less independent, though they unite to form a continuous 

 septum. 



The septum of the truncus arteriosus arises on the fifth day 

 as a complete partition extending from the cephalic border of 

 the two pulmonary arches into the upper portion of the bulbus 

 arteriosus; the blood current flowing through the bulbus that 

 passes behind this partition enters the pulmonary arches exclu- 

 sively, that passing in front enters the two remaining pairs of aortic 

 arches. During the latter half of the fifth day and on the sixth 

 day the septum of the truncus is continued into the proximal por- 

 tion of the bulbus and divides it in two stems. Here, however, 

 it co-operates with three longitudinal ridges of the endocardium 

 of the bulbus, one of which is in tlie direct line of prolonga- 

 tion of the septum of the truncus, which therefore is continued 

 along this one and between the other two as far as the place of 

 formation of the semilunar valves (Fig. 201). The entire septum 

 thus formed has a slightly spiral course, of such a nature that 



