560 EMBRYOLOGY. 



Schmidt. The ridges have arisen from a growth of the endocardium, 

 and consist of a gelatinous connective substance and an endothelial 

 investment. The atrial partition, when it has grown down to the 

 auricular canal, soon fuses along its free lower margin with these 

 lips (fig. 309 si) ; the auricular canal is thereby divided into a left 

 and a right atrioventricular opening, — ostium atrioventriculare 

 sinistrum and dextrum (fig. 310 B F.av.s and F.av.d), — and at 

 the same time both the dorsal and ventral endocardial ridges, which 

 originally bound the opening, are divided in the middle {p.ek and u.eh). 

 The dorsal components soon fuse with the corresponding pieces of 

 the opposite [ventral] side, and thus there arise at the lower margin 

 of the atrial partition (fig. 309 si) two new ridges, — one of which 

 projects into the left, the other into the right atrioventricular 

 opening, — which furnish the foundation of the median cuspidate 

 valves. 



The development of the atrial partition and the division of the 

 auricular canal into the two atrioventricular openings are closely 

 related processes, the former being the cause of the latter. This 

 is clearly proved by pathological -anatomical conditions of arrested 

 development of the heart. In all cases in which the formation of 

 the atrial partition has been for any reason whatever interrupted 

 and the lower part of it has been altogether wanting, there has 

 always been only one atrioventricular opening (an ostium venosum 

 commune) present (Arnold). 



Before we progress further in the history of the development of 

 the atrium, we must add an account of the metamorphoses which 

 have taken place meanwhile in the territory of the ventricle and 

 truncus arteriosus. 



The ventricle begins to acquire its partition not much later than 

 the atrium. By the end of the first month its musculature has 

 become considerably thickened (fig. 311 A). Muscular trabecule 

 have arisen, which project far into the interior of the chamber and 

 are joined to one another, so as to constitute a spongy tissue, the 

 numerous fissures in which are continuous with the narrowed cavity 

 of the heart and likewise allow the current of the blood to pasc 

 through them. At one place the musculature is especially thickened 

 and forms a crescent-jhaped fold projecting inward, the fundament 

 of the ventricular j>artition (septum ventriculorum) (figs. 308, 309, 

 310 ks). This takes its origin from the lower and posterior [dorsal] 

 wall of the ventricle, in the region which is marked externally by 

 the previously mentioned sulcus interventricularis (fig. 307 si). Its 



