HEART. 



213 



primary septum forms a valvular flap which closes the foramen from the side 

 of the left auricle. In foetal life blood passes from the right to the left auricle 

 through the foramen ovale ; but at birth, with the establishment of respiration, 

 the valvular septum primum unites with the septum secundum and the parti- 

 tional wall is complete. The annulus ovalis represents the edge of the septum 

 secundum. 



By the end of the first month the left venous valve and the septum secundum 

 seem to have united with one another to obliterate a space seen in earlier stages 

 between the left venous valve and the 

 septum of the auricles. Further the 

 right venous valve is continuous with 

 the posterior horn of the septum as it 

 passes to join the septum primum at 

 the base of the posterior endocardial 

 cushion. The sinus venosus thus 

 comes to lie partly in the septum 

 secundum as it passes forward to 

 open into the auricle (Low). 1 The 

 slit-like opening of the sinus venosus 

 meanwhile opens out, and the cavity 

 o,f the sinus is completely taken into 

 that of the auricle, the junction of 

 the two chambers being indicated 

 merely by the sulcus terminates of His. 

 The greater part of the right venous 

 valve is converted into the Eustachian 

 valve, but its lower edge becomes the 

 valve of Thebesius. 



The left auricle receives, at a 

 stage when the primary septum is 

 appearing, the common stem of the 

 right and left pulmonary veins con- 

 veyed to it in the dorsal mesocar- 

 dium (figs. 266, 267). In later stages 

 the mouth of the vessel opens up 

 like the mouth of the sinus venosus, 

 and its proximal part is taken into the 

 auricle as its atrium. This process of 

 expansion proceeds in some animals 

 until the union of the two pulmonary 

 veins is reached, so that they come to 

 open separately into the chamber ; but 

 in man it is carried still farther, and 



PIG. 267. SECTION THROUGH THE HEART OF A 



RABBIT-EMBRYO. (Bom.) 



r.s., l.s., right and left horns of sinus receiving 

 from above the respective ducts of Cuvier ; r.au., 

 l.au., right and left auricles ; r.v. t Lv., right and 

 left parts of the ventricle ; r.v.v., l.v.v., right and 

 left valves guarding the orifice from the right horn 

 of the sinus into the right auricle ; au.v.c., one of 

 the two endocardial cushions which are beginning 

 to subdivide the common auriculo-ventricular aper- 

 ture. The dotted line encloses the extent of the 

 endocardial thickening. The small oval detached 

 area of endocardial thickening in the right ventricle, 

 and the swelling opposite it on the septum inferius, 

 belong to the proximal chamber of the aortic bulb, 

 and will afterwards unite to separate the conus of the 

 right from the aortic vestibule of the left ventricle, 

 s', septum primum growing down between the 

 auricles and prolonged below by a thickening of 

 endocardium. Close to this septum in the left 

 auricle is seen the opening of the pulmonary vein ; 

 s.m/., inferior septum of the ventricles. 



extends to the junction of the two 



main tributaries of each vein, in consequence of which the auricle comes to 



have four separate pulmonary veins opening into it. 



Auricular canal. By the overlapping of the expanding auricular and 

 ventricular chambers the auricular canal is telescoped within the cavity of the 

 ventricle. Its lumen becomes slit-like, and endocardial cushions develop on its 

 dorsal and ventral walls. These fuse with one another to divide the canal into two 

 passages, which become the auriculo-ventricular openings. When the auricular 



Proc. Anat. and Anth. Soc. Univ. of Aberdeen, 1900-1902. 



