vii THE STAGES OF LIFE AND DEATH 265 



which allows the blood returning by the venae cavae to pass 

 from the right to the left side of the heart (Fig. 116). Whereas 

 in the adult there exists no communication between the system 

 of pulmonary vessels or little circulation and the system of aortic 

 vessels or great circulation, in the foetus the system of pulmonary 

 vessels communicates freely with the aortic system by means of 

 a canal, also known to Galen, called the ductus arteriosus Botalli. 

 It springs from the pulmonary artery before its division into two 

 pulmonary branches, and opens into the aorta beyond the origin 

 of the innominate, the common carotid, and the left sub-clavian 

 (Fig. 117). In addition, the foetal placenta adherent to the 

 uterine mucosa is connected by arterial and venous vessels with 

 the vascular system of the foetus. The two umbilical arteries 

 arise in the hypogastrics or internal iliacs, pass through the cord, 

 and branching up, form the capillaries of the placenta, from 

 which arises the umbilical vein ; this vein passes through the 

 cord in the opposite direction (Fig. 118) and divides into two 

 branches directly it enters the abdomen of the foetus, one of these 

 branches, the ductus venosus Arantii, running into the inferior 

 vena cava, the other distributing itself in the liver (Fig. 119). 

 After these anatomical preliminaries we can follow the course of 

 the foetal blood stream and note the changes undergone by the 

 blood during its passage from the foetus to the placenta and 

 thence to the foetus. 



The blood brought to the placenta by the two umbilical 

 arteries is distinctly venous, that is, it is poor in oxygen and 

 rich in carbon dioxide and products formed by the respiration and 

 the nutritive exchanges of the foetal tissues. When it reaches 

 the placental capillaries, it enters into relation with the maternal 

 arterial blood circulating in the uterus so that a double change in 

 opposite directions takes place : the maternal arterial blood becomes 

 venous and the placental venous blood becomes arterial, after dis- 

 charging carbon dioxide and waste products, and absorbing 

 oxygen and nutritive materials. The umbilical vein thus brings 

 back to the foetus blood which is distinctly arterial. Before, 

 however, reaching the heart of the foetus it mingles with the 

 venous blood returning from the foetal extremities to the heart 

 by the inferior vena cava, so that it becomes less arterial. Only 

 part of this arterial blood goes to the liver, where it mixes with 

 the venous blood of the portal vein. At the point where the 

 hepatic veins coming from the liver open into the inferior vena 

 cava the arterial blood which the cava contains undergoes a 

 second diminution of its arterial character, so that the blood 

 which reaches the heart of the foetus is less arterial than that 

 contained in the umbilical vein. 



When this blood reaches the right auricle, it mingles with 

 the venous blood flowing from the upper limbs and the head 



