DEVELOPMENT OF THE HEART. 633 



and yellow for the inferior. The two syringes were managed, at 

 the same time, by the right and left hands : their nozzles being 

 firmly held in place by the fingers of an assistant. \Yhen the 

 points of the syringes were introduced into the veins, at equal dis- 

 tances from the heart, and the two injections made with equal force 

 and rapidity, it was found that the admixture of the colors which 

 took place was so slight, that at least nineteen-twentieths of the 

 yellow injection had passed into the left auricle, and nineteen-twen- 

 tieths 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, however 

 contrary to what was found by Dr. Keid, there was always an ad- 

 mixture of the two colors, generally in about equal proportions. 

 This discrepancy may be owing to the smaller size of the head and 

 upper extremities, in the pig, as compared with those of the human 

 subject, which would prevent their receiving all the blood coming 

 from the left ventricle ; or to some differences in the manipulation 

 of these experiments, in which it is not always easy to imitate ex- 

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

 the living foetus. The above results, however, are such as to leave 

 no doubt of the principal fact, viz., 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 three 

 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, therefore, 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 and distinct at the end of the sixth month 

 (Fig. 271), subsequently becomes atrophied to such an extent that, 

 at the end of gestation, it has altogether disappeared, or is at least 

 reduced to the condition of a very narrow, almost imperceptible 



