DEVELOPMENT OF THE VASCULAR SYSTEM. 



805 



Fig. 315. 



Diagram of the CIRCULATION 



THROUGH THE FtETAL HEART,. 



a. Superior vena cava. b. Inferior 

 vena cava. c, c, c, c. Arch of the aorta 

 and its branches. d. Pulmonary 

 artery. 



from the arch of the aorta, before its junction with the ductus arteriosus ; 

 and this arrangement causes the blood of the two venae cavae, not only 

 to enter the heart in different directions, but also to be distributed, after 

 leaving the ventricles, to different parts of the body. (Fig. 315.) The 

 blood of the superior vena cava passes 

 through the right auricle downward into 

 the right ventricle, thence through the 

 pulmonary artery and ductus arteriosus, 

 into the thoracic aorta ; while the blood 

 of the inferior vena cava, entering the 

 left auricle, passes into the left ventricle, 

 thence into the arch of the aorta, and is 

 distributed to the head and upper extre- 

 mities. The two streams, therefore, in 

 passing through the heart, cross each 

 other both behind and in front. The 

 venous blood, returning from the head 

 and upper extremities by the superior 

 vena cava, passes through the thoracic 

 and abdominal aorta and the umbilical 

 arteries, to the lower part of the body, 

 and to the placenta ; while that returning 

 from the placenta, by the inferior vena 



cava, is distributed to the head, and upper extremities, through the 

 vessels given off from the arch of the aorta. 



This division of the streams of blood, during a certain period of foetal 

 life, is so complete that Reid, 1 on injecting the inferior vena cava with 

 red, and the superior with yellow, in a human foetus of seven months, 

 found that the red injection had passed through the foramen ovale into 

 the left auricle and ventricle and the arch of the aorta, and had filled 

 the vessels of the head and upper extremities ; while the yellow had 

 passed into the right ventricle, pulmonary artery, ductus arteriosus, and 

 thoracic aorta, with only a slight admixture of red at the posterior part 

 of the right auricle. All the branches of the thoracic and abdominal 

 aorta were filled with yellow, while the whole of the red had passed to 

 the upper part of the body. 



We have repeated this experiment several times on the foetal pig, 

 when about one-half and three-quarters grown, first taking the precau- 

 tion to wash out the heart and large vessels with a watery injection, 

 immediately after the removal of the foetus, and before the blood had 

 been allowed to coagulate. The injections used were blue for the supe- 

 rior vena cava, and yellow for the inferior. The two syringes were 

 managed, at the same time, by the right and left hands ; their nozzles 

 being held in place b}' the fingers of an assistant. When the points of 

 the syringes were introduced into the veins, at equal distances from the 



Edinburgh Medical and Surgical Journal, 1835, vol. xliii. p. 11. 



