CIRCULATION IN SPECIAL PARTS 249 



that enters the vena cava is purely arterial in character; but, being 

 mixed in the vessels with the venous blood returned from the trunk 

 and lower extremities, it loses this character in some degree by the 

 time it reaches the heart. In the right auricle, which it then enters, 

 it would also be mixed with the venous blood brought down from the 

 head and upper extremities by the descending vena cava, were it not 

 that a provision exists to impede (if it does not entirely prevent) any 

 further admixture. This consists in the arrangement of the Eustachian 

 valve, which directs the arterial current (that flows upwards through 

 the ascending vena cava) into the left side of the heart, through the 

 foramen ovale an opening in the septum between the auricles 

 whilst it directs the venous current (that is being returned by the 

 superior vena cava) into the right ventricle. 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 extremities before it undergoes any further admixture; 

 while the venous blood contained in the right ventricle is forced into 

 the pulmonary artery, and thus through the d.uctus arteriosus 

 branching off from the pulmonary artery before it passes to the two 

 lungs into the descending aorta, mingling with the arterial currents 

 which that vessel previously conveyed, and thus supplying the trunk 

 and lower extremities with a mixed fluid. A portion of this is con- 

 veyed by the umbilical arteries to the placenta, in which it undergoes 

 the renovating influence of the maternal blood, and from which it is 

 returned in a state of purity. In consequence of this arrangement 

 the head and upper extremities are supplied with pure blood returning 

 from the placenta, whilst the rest of the body receives blood that is 

 partly venous. This is probably the explanation of the fact that 

 the head and upper extremities are most developed, and from their 

 weight occupy the inferior position in the uterus. At birth the course 

 of the circulation undergoes changes. As soon as the lungs are dis- 

 tended by the first inspiration, a portion of the blood of the pulmonary 

 artery is diverted into them and undergoes aeration; and, as this 

 portion increases with the full activity of the lungs, the ductus arteri- 

 osus gradually shrinks, and its cavity finally becomes obliterated. 

 At the same time the foramen ovale is closed by a valvular fold, and 

 thus the direct communication between the two auricles is cut off. 

 When these changes have been accomplished, the circulation, which 

 was before carried on upon the plan of that of the higher reptiles, 

 becomes that of the complete warm-blooded animal, all the blood 

 which has been returned in a venous state to the right side of the heart 

 being transmitted through the lungs before it can reach the left side- 

 or be propelled from its arterial trunks. 



After birth the umbilical arteries shrink and close up, and become 

 the lateral ligaments of the bladder, while their upper parts remain 

 as the superior vesical arteries. The umbilical vein becomes the 

 ligament-urn teres. The ductus venosus also shrinks, and finally is 

 closed. The foramen ovale is closed, and the ductus arteriosus shrivels 

 and becomes the ligamentum arteriosum. 



