FCETAL CIRCULATION. 



465 



enters the vena cava is purely arterial 

 in its character ; but being mixed in 

 the vessels with the venous blood that 

 is returned from the trunk and lower 

 extremities, it loses this character in 

 some degree, by the time that it 

 reaches the heart. In the right 

 auricle, which it then enters, it would 

 also be mixed with the venous blood 

 which is brought down from the head 

 and upper extremities by the de- 

 scending cava ; were it not that a 

 very curious provision exists, to im- 

 pede (if it does not entirely prevent) 

 any further admixture. This con- 

 sists in the arrangement of the Eusta- 

 chian valve, which directs the arterial 

 current (that flows upwards through 

 the ascending cava) into the left side 

 of the heart, through the foramen 

 ovale, whilst it directs the venous 

 current (that is being returned by the 

 descending cava) into the right ven- 

 tricle. When the ventricles contract, 

 the arterial blood contained in the 

 left is propelled into the ascending 

 Aorta, and supplies the branches that 

 proceed to the head and upper ex- 

 tremities, before it undergoes any 

 further admixture ; whilst the venous 

 blood contained in the right ventricle, 

 is forced into the Pulmonary artery, 

 and thence through the ductus arte- 

 riosus (17), which is like a continua- 

 tion of its trunk, into the descending 

 aorta, mingling with the arterial cur- 

 rent which that vessel previously con- 

 veyed, and thus supplying the trunk 

 and lower extremities with a mixed 

 fluid. A portion of this is conveyed, 

 by the umbilical arteries, to the Pla- 

 centa ; in which it undergoes the 

 renovating influence of the maternal 

 blood, and from which it is returned 

 in a state of purity. 



824. Hence the head and superior 

 extremities, whose development is 

 required to be in advance of that 

 of the lower, are supplied with blood 

 nearly as pure as that which returns 

 from the placenta ; whilst the rest 



30 



Fig. 150. 



The Foetal Circulation : 1. The umbilical cord, 

 consisting of the umbilical vein and two um- 

 bilical arteries; proceeding from the placenta 

 (2). 3. The umbilical vein dividing into three 

 branches; two (4, 4) to be distributed to the 

 liver ; and one (5), the ductus venosus, which 

 enters the inferior vena cava (6). 7. The portal 

 vein, returning the blood from the intestines, and 

 uniting with the right hepatic branch. 8. The 

 right auricle ; the course of the blood is denoted 

 by the arrow, proceeding from 8, to 9, the left 

 auricle. 10. The left ventricle; the blood fol- 

 lowing the arrow to the arch of the aorta (11), 

 to be distributed to the branches given off by the 

 arch to the head and upper extremities. The ar- 

 rows 12 and 13, represent the return of the blood 

 from the head and upper extremities through 

 the jugular and subclavian veins, to the supe- 

 rior vena cava (14), to the right auricle (8), and 

 in the course of the arrow through the right 

 ventricle (15), to the pulmonary artery (16). 17. 

 The ductus arteriosus, which appears to be a 

 proper continuation of the pulmonary artery; 

 the offsets at each side are the right and left 

 pulmonary artery cut off; these are of extremely 

 small size as compared with the ductus arterio- 

 sus. The ductus arteriosus joins the descending 

 aorta (18, 18), which divides into the common 

 iliacs, and these into the internal iliacs, which 

 become the hypogastric arteries (19), and return 

 the blood along the umbilical cord to the pla- 

 centa; while the other divisions, the external 

 iliacs (20), are continued into the lower extremi- 

 ties. The arrows at the terminations of these 

 vessels mark the return of the venous blood by 

 the veins to the inferior cava. 



