THE HEART. 



569 



Seen from within, the auricular portion of the heart has, at 

 the end of the fourth week, the appearance shown in Fig. 244. 

 Opposite the external constriction, a fold, sk, the septum superius, 

 projects into the cavity from its anterior end and ventral wall, and 

 reduces the communication between the two auricles to a rather 



Fig. 244. — The dorsal half of the heart of a Human Embryo, twenty-eight 

 days old, seen from within. The heart has been bisected lengthways, and 

 the ventral half removed. (From His.) x 32. 



EA, right auricle. EB, left auricle. EP, auriculo-ventricular aperture. EK, 

 ctinalis auriculari s. ES, opening of sinus venosus into right auricle. EV, right 

 ventricle. EY, left ventricle. SB, septum spurium. SD, septum inferius. SK, 

 septum superius. SZ, spina vestibuli. VD, right vena cava anterior. VU, Eustachian 



valve. 



small circular aperture, nearer the dorsal than the ventral 

 surface. 



The conspicuous projection into the dorsal part of the right 

 auricle, shown in the figure, is caused by the sinus venosus. 

 The aperture from the sinus venosus into the auricle is an 

 obliquely placed slit, RS, of which the outer lip is thickened, and 

 forms the Eustachian valve, VU ; while the opposite, or inner lip, 

 is a thin fold, which at the lower end of the slit passes into a 

 triangular thickening of connective tissue, the spina vestibuli, 



