THE COUESE OF THE CIRCULATION. 583 



openings, and in slightly older foetuses all four openings are 

 present ; the change being apparently due to the opening out 

 of the originally single orifice, and the absorption of the vein, as 

 far as its first branches, into the wall of the auricle ; much in the 

 same way as the sinus venosus is opened out, and made part of 

 the wall of the right auricle. 



4. The Course of the Circulation during the first Four Months of 

 Gestation. 



In the early stages, up to the end of the first month, the 

 blood brought back to the heart whether from the body of the 

 embryo itself, from the placenta, or from the yolk-sac is poured 

 into the sinus venosus, and thence, through a median slit-like 

 aperture, into the single auricular cavity. Complete mixture of 

 the streams from the several sources must necessarily occur, in 

 both the sinus venosus and the auricle, and the blood driven out 

 through the truncus arteriosus by the ventricle will be of a 

 mixed character. 



After the sinus venosus is taken into the heart, in the early 

 part of the second month, there are for a time three separate 

 openings into the right auricle : those of the right and left 

 Cuvierian veins, and of the posterior vena cava respectively. The 

 auricular septum is now partially formed, but there is still free 

 communication between the two auricles through the foramen 

 ovale. Of the three veins, the opening of the posterior vena 

 cava lies nearest to the foramen ovale ; and the Eustachian 

 valve, a fold of the wall of the auricle along the right-hand side 

 of the opening, tends to direct the blood from the posterior vena 

 cava through the foramen ovale into the left auricle. The 

 foramen ovale is at this stage a mere aperture in the auricular 

 septum, not guarded by valves, so that a certain amount of direct 

 mixture of the blood returned to the auricle by the different 

 veins must of necessity take place. 



During the third month, the transverse communication from 

 the left to the right Cuvierian vein is being established ; and 

 by the end of the fourth month the left Cuvierian vein has 

 practically disappeared, the whole of the blood from both sides 

 of the head, and from both fore limbs, being returned by the 

 right Cuvierian vein, or anterior vena cava as it may now be 

 called. Neglecting the coronary sinus, which is comparatively 



