80(3 DEVELOPMENT OF THE VASCULAR SYSTEM. 



heart, and the two injections made with equal rapidity, it was found 

 that the admixture of the colors was so slight, that at least nineteen- 

 tvventieths of the yellow injection had passed into the left auricle, and 

 nineteen-twentieths of the blue into the right. The pulmonary artery 

 and ductus arteriosus contained a similar proportion of blue, and the 

 arch of the aorta of yellow. In the thoracic and abdominal aorta, how- 

 ever, there was always an admixture of the two colors, generally in 

 about equal proportions. This may be owing to the smaller size of the 

 head and upper extremities in the pig, as compared with those of 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 exactly 

 the force and rapidity of the different currents of blood in the living 

 bod}'. These results, however, leave no doubt of the fact, that, up to 

 an advanced stage of foetal life, by far the greater portion of the blood 

 coming from the inferior vena cava passes through the foramen ovale, 

 into the left side of the heart; while by far the greater portion of that 

 coming from the head and upper extremities 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 lungs increasing in size, the two pulmonary arteries, as well 

 as the pulmonary veins, enlarge in proportion ; and a greater quantity 

 of the blood coming from the right ventricle, instead of going onward 

 through the ductus arteriosus, passes to the lungs, and, returning thence 

 by the pulmonary veins 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 

 atrophied to such an extent that, at the end of gestation, it has either 

 disappeared, or is reduced to the condition of a narrow membranous 

 ridge, which can exert no 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 ; and a passage of blood from 

 one to the other may more readily take place. 



Thirdly, the foramen ovale becomes partially closed by a valve which 

 passes across its orifice from behind forward. This valve, which begins 

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

 It consists of a thin, fibrous sheet, which grows from the posterior sur- 

 face of the auricular cavity, a little to the left of the foramen ovale, and 

 projects into the left auricle, presenting a thin crescentic border, at- 

 tached, b}' its two extremities, to the auricular septum upon the left 

 side. The valve does not at first interfere with the flow of blood from 

 right to left, since its edge hangs loosely into the cavity of the left 

 auricle. It only opposes regurgitation from left to right. 



But as gestation advances, while the walls of the heart continue to 

 enlarge, and its cavities expand in every direction, the fibrous bundles, 



