240 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



about the 6 th week, and are fused together by a thin mem- 

 brane — the pars membranacea septi (Fig. 198). It is seen 

 under cover of the septal segment of the tricuspid valve and 

 is the last part of the septum to form ; occasionally it fails 

 to form, and a communication exists between the ventricles. 

 This condition is commonly associated with a partial failure in 

 the formation of the pulmonary artery. Such a condition may 

 give rise to cyanosis ; the subjects of it commonly die before 15 of 

 pulmonary tuberculosis. The walls of the right and left ventricle 

 in such cases are of equal thickness, for both act as common 

 systemic and pulmonary pumps. 



(4) The septum primum of the auricle springs from the 

 posterior wall of the primitive auricle, and falls like the blade 

 of a guillotine on the endocardial cushions with which it fuses 

 (Figs. 197 and 198). Its upper border separates from the 

 auricular wall. It forms the septum ovale. As it descends 

 it involves and fuses with the left venous valve. 



(5) The septum secundum grows down from the roof and 

 posterior wall of the primitive auricle to the right of the 

 septum primum. It forms the annulus ovalis and the auricular 

 septum above it (Fig. 198). 



The foramen ovale (Fig. 198) is an interval left between 

 the two auricular septa ; the Eustachian valve becomes connected 

 with the annulus (septum secundum). In 75 per cent, of 

 people, following Fawcett's statistics, the foramen becomes closed 

 in the first year after birth. In 2 5 per cent, it remains partially 

 open, but even when an opening remains the blood could pass 

 from the right to the left auricle only when the pressure within 

 the right auricle is greater than within the left. The valvular 

 action of the septum ovale prevents the passage of blood from 

 the left to the right auricle. The foramen ovale is an adapta- 

 tion to the foetal form of circulation. 



Thus the primitive auricle is divided into right and left 

 chambers by two septa — septum 'primum and secundum ; the 

 bulbus arteriosus by the anterior and posterior aortic septa ; 

 the auriculo-ventricular canal by the anterior and posterior endo- 

 cardial cushions; 'and, lastly, the primitive ventricle into right 

 and left by the inter-ventricular septum. 



Tricuspid and Mitral Valves.— It is to be remembered 



