212 



VASCULAK SYSTEM 



through the auricular canal. It becomes distended at a very early stage into two 

 lateral diverticula, which become the auricular appendages (fig. 265).- On the 

 outer side of the connecting portion between these appendages (the original 

 tubular auricle) a fold now appears which indicates the position of the future 

 primary septum. On the inner aspect another fold is also now seen running over 

 the roof and connected with the venous valves. It is known as the septum 

 spurium, and corresponds to a muscular fold which stretches these valves in the 

 reptilian heart (Kose).- 



Immediately to the left of this the primary septum extends inwards from the 

 dorsal wall (fig. 266); This is the septum superius of His, or septum primum of 



left ven. valve 



septum primum 



right ven. valve ^ 



right auricle 



left auricle 



left 

 ventricle 



FIG. 266. MODEL OF THE HEAET OF A HUMAN EMBBYO OF 6-8 MM. (After Piper.) 



. The primitive auricule has been opened up to show the septum primum and the valves guarding 

 the mouth of the opening of the sinus venosus. The fold passing on to the roof of the auricle and 

 connected with the venous valves is the septum spurium. 



Born. At first it .forms only an incomplete partition, the two sides of the 

 common auricle communicating below its free thickened lower border by the 

 ostium primum (Born) (fig. 265). It soon fuses below with the endocardial 

 thickening in the auricular canal (see below), and the ostium primum is closed, but 

 a new opening (ostium secundum) appears near its dorsal attachment (fig. 268). 

 Meantime a second septum (septum secundum, Born) has extended into the 

 cavity. It is sickle-shaped, and the horns of its free semilunar border fuse with 

 the ventral attachments of the septum primum. It overlaps the primary 

 septum and the opening between the two is the foramen ovale. The aperture 

 is bounded by the free border of the secondary septum, and the free portion of the 



