360 C. V. MORRILL 



extensively. He rejected the idea that the septum intermedium 

 is concerned in the formation of the Hmbus or annulus ovaUs. 

 He also discarded the term 'septum musculare,' calling it simply 

 the limbus (Vieussenii) and described it as arising in the anterior 

 upper atrial wall as an infolding caused by the truncus arterio- 

 sus (bulbus) imbedding itself between the two atria. From here 

 it spreads along the upper and then along the lower wall of the 

 right atrium. It contains a connective tissue core which was 

 pinched off from the surface of the truncus during the process 

 of infolding. The presence of this core, Rose thinks, is a proof 

 that infolding has occui*red. 



In the pig the upper part of septum II or limbus which cor- 

 responds to the curve of the bulbus, is at first a broad thicken- 

 ing of the musculature of the atrial wall which in later stages 

 becomes more prominent. Only in its lower anterior part or 

 root are connective tissue elements found. These are the re- 

 mains of the original thickened endocardium which was show^n 

 to form the basis for the septum. Retzer ('08) in denying the 

 presence of a septum II in the pig says: "This supposed septum 

 which His correctly described as a 'muskulose Leiste' is nothing 

 but a fold in the atrial wall at that place." It is formed by 

 the atrial growing around the conus arteriosus as a fixed point, 

 thus causing a bulging inwards of the atrial wall. 



The descriptions of Rose and Retzer thus correspond quite 

 closely as far as the origin of the muscular fold is concerned. 

 Retzer, however, thinks it never attains sufficient size to be 

 called a septum. His ('85) described the same structure, call- 

 ing it the anterior septum or 'sickle' which later gives rise to 

 the limbus Vieussenii. He thought that it was formed by the 

 septum intermedium growing up on the anterior atrial wall to 

 meet the anterior end of the septum superius (septum I). This 

 upgrowth of the septum intermedium, or a portion of it over- 

 laid by muscle, is practically what takes place in the pig, but 

 as pointed out previously, there is no evidence of an infolding 

 of the atrial wall at a higher level. 



Favaro's ('13) recent account of the septum secundum in the 

 guinea-pig and sheep differs very little from that given above 



