COURSE OF THE CIRCULATION IN THE F(ETUS. 941 



Ca\\T superiores, entering the right auricle separately ; but in the higher 

 Mammalia and in Man, the left duct is obliterated, and the right alone re- 

 mains as the single Vena Cava superior, a transverse communicating branch 

 being formed, to bring to it the blood of the left side. 1 The double Vena 

 Cava sometimes presents itself as a monstrosity in the Human subject. As 

 the anterior extremities are developed, the subclavian veins are formed to 

 return the blood from them; and these discharge themselves into the jug- 

 ulars. The Omphalo-mesaraic vein (Fig. 344, q), which has been already 

 ( 776) shown to be formed by the confluence of the veins of the yolk-bag 

 and intestinal canal, passes by itself, with the two Cuvierian ducts, into the 

 auricle. The upper part of this remains to constitute the upper part of the 

 Inferior Cava (Figs. 344, 345, j), the lower portion of which arises between 

 the Wolffian bodies, and originally enters the omphalo-meseuteric vein 

 above the liver. The Inferior Cava, which receives the hepatic vein, is 

 gradually enlarged by the reception of most of the veins from the inferior 

 part of the trunk and the lower extremities, and the Vena Azygos is re- 

 duced in the same proportion ; in some rare cases of abnormal formation, 

 however, the vena cava fails to be developed, and then the blood from the 

 lower parts of the body is conveyed to the superior cava through the system 

 of the vena azygos. 



783. The following is the course of the Circulation in the mature Foetus. 

 The fluid brought from the Placenta by the umbilical vein, is partly con- 

 veyed at once to the ascending Cava by means of the ductus venosus, but 

 chiefly flows through the vena portse into the Liver, whence it reaches the 

 ascending Cava by the hepatic vein. Having thus been transmitted through 

 the great depurating organ, the Placenta, and the great assimilating organ, 

 the Liver, 2 it is in the condition of arterial blood ; but, being mixed in the 

 great vessels with that which has been returned from the trunk and lower 

 extremities, it loses this character in some- degree by the time that it arrives' 

 at the Heart. In the right auricle, which it then enters, it would be also 

 mixed with the venous blood brought thither by the descending Cava; were 

 it not that a very curious provision exists to prevent (in great degree, if not 

 entirely) any such further dilution. The Eustachiau valve has been found, 

 by the experiments of Dr. J. Reid, 3 to serve the purpose of directing the 

 arterial blood, which flows upwards from the ascending Cava, through the 

 foramen ovale into the left auricle, whence it passes into the left ventricle; 

 whilst it also directs the venous blood, that has been returned by the descend- 

 ing Cava, into the right ventricle. When the ventricles contract, the arte- 

 rial blood which the left contains is propelled into the ascending Aorta, and 

 supplies the branches that proceed to the head and upper extremities, before 

 it undergoes any admixture ; whilst of the venous blood contained in the 

 right ventricle, part is transmitted by the Pulmonary artery to the lungs, 

 but another (and probably by far the larger) part finds its way through the 

 Ductus Arteriosus into the descending Aorta, mingling with the arterial 

 current which that vessel previously conveyed, and passing thus to the 

 trunk and lower extremities. Hence the head and superior extremities, 



1 Seethe elaborate Memoirs, On the Development of the Great Anterior Veins of 

 Man and Mammalia (Phil. Trans., 185"), by Mr. J. Marshall; who has further shown 

 that some vestiges of the original arrangement may be traced even in the normal 

 condition of the venous system in the adult. 



2 It does not seem probable that the depurating action of the Liver can be ener- 

 getically performed during foetal life; and its large dimensions and copious supply of 

 blood appear rather to be referable to its (unction as a blood-making gland. 



3 Edinb Med. and Surg. Journal, vol. xliii ; and Anat., Pbysiol., and Pathol. 

 Researches, chap. ix. 



