262 CLINICAL APPLIED ANATOMY. 



which persists as the foramen ovale. The ostium secundum is 

 closed by the downgrowth of a secondary septum from the roof 

 and posterior part of the auricle. This new septum, which lies 

 to the right of the septum primum, is known as the septum 

 secundum. Its curved lower edge, applied to the right side of 

 the septum primum, forms the annulus ovalis, and thus comes 

 to bound the foramen ovale above, whilst the septum primum 

 bounds it below. Since the septum secundum restores the 

 integrity of the upper part of the auricular partition, defects in 

 this position are attributed to failures in the development of 

 the septum in question. 



Patency of the foramen ovale is due to a failure of development 

 of that part of its membranous floor which is derived from the 

 septum primum. An oblique valvular slit is frequently found 

 at the margin of the fossa ovalis, and is caused by failure of the 

 membranous floor to fuse with the annulus. Such an opening 

 is kept closed during life by the excess of the blood pressure in 

 the left auricle over that in the right. 



Defects in the ventricular septum were at one time supposed 

 to be always situated at the site of the pars membrancea septi or 

 undefended space, but this is not strictly true. The ventricular 

 septum is of compound origin, being formed above by the fusion 

 of the endocardial cushions of the auricular canal with those of 

 the bulbus arteriosus and below by the septum inferius which 

 separates the primitive ventricle into two, and, growing upwards, 

 fuses with the endocardial cushions. The septum inferius 

 becomes attached to the anterior cushion of the bulbus in front 

 and with the posterior cushion of the auricular canal behind. 

 Between these two attachments lies the interventricular foramen. 

 Quite early in foetal life this foramen becomes closed by fusion 

 of its margins, and its site is indicated by the undefended space 

 of the fully formed heart. 



The upper part of the completed ventricular septum, in front 

 of the undefended space, forms the septal wall of the infundi- 

 bulum of the right ventricle, and is often known as the aortic 

 septum. Indications of the compound nature of this portion of 



