DEVELOPMENT OF THE VASCULAR SYSTEM. 701 



head and upper extremities in the pig, as compared with the human 

 foetus, which would prevent their receiving all the blood coming from 

 the left ventricle ; or to some difference in the manipulation of these 

 experiments, in which it is not always easy to imitate the force and 

 rapidity of the blood currents in the living body. The results, however, 

 leave no doubt that, up to an advanced stage of fatal life, most of the 

 blood from the inferior vena cava passes through the foramen ovale into 

 the left side of the heart ; while most of that coming from the head and 

 upper limbs passes into the right side of the heart, and thence outward 

 by the pulmonary trunk and ductus arteriosus. Toward the latter 

 periods of gestation, this division of the venous currents becomes less 

 complete, owing to the following causes : 



First, the two pulmonary arteries, as well as the pulmonary veins, 

 enlarge in proportion to the increased size of the lungs ; and a greater 

 quantity of blood from the right ventricle, instead of passing through 

 the ductus arteriosus, is distributed to the lungs, and, returning thence 

 to the left auricle and ventricle, joins the stream passing out by the arch 

 of the aorta. 



Secondly, the Eustachian valve diminishes in size. This valve, which 

 is very large at the end of the sixth month, subsequently becomes atro- 

 phied, and at the end of gestation is too small to exert any influence on 

 the current of the blood. Thus, the cavity of the inferior vena cava, 

 at its upper extremity, ceases to be separated from that of the right 

 auricle. 



Thirdly, the foramen ovale becomes partially closed by a valvular 

 partition growing from behind forward. This valve, which begins to 

 be formed at a very early period, is the valve of the foramen ovale. It is 

 a thin, fibrous sheet, attached to the posterior surface of the auricular 

 cavity a little to the left of the foramen ovale, and projecting by its free 

 border into the left auricle. It accordingly does not interfere at this 

 time with the flow of blood from right to left, and only prevents regur- 

 gitation from left to right. 



But as gestation advances, while the heart continues to enlarge, and 

 its cavities expand in every direction, the fibrous bundles, forming the 

 valve do not elongate in proportion. The valve, accordingly, becomes 

 drawn down more closely across the foramen ovale. It thus comes in 

 contact with the inter-auricular septum, and unites with its substance ; 

 the adhesion taking place first at its lower and posterior portion, and 

 extending gradually upward and forward, so that the passage from the 

 right auricle to the left becomes more oblique. 



At the same time the inferior vena cava alters its position. This 

 vessel, which at first looked transversely toward the foramen ovale, 

 turns partially forward ; and as the Eustachian valve has now nearly 

 disappeared, some of the blood from the inferior vena cava enters the 

 right auricle, while the remainder still passes through the foramen 

 ovale. 



