3 o 4 THE VASCULAR SYSTEM 



common trunk, in the adult they remain enveloped by the 

 same tubular sheath of serous pericardium (p. 288). 



The Circulation in the Foetus. The chambers of the 

 foetal heart communicate with precisely the same blood-vessels 

 as they do in the adult, but these vessels have, in some 

 instances, rather different duties to perform. 



The pure blood returns from the placenta by the umbilical 

 vein, which joins the left branch of the portal vein (Fig. no), 

 and, if there were no " short circuit," it would require to pass 

 through the liver before reaching the heart. A certain amount 

 of the pure blood does enter the liver, and this fact accounts 

 for the large size of that viscus in the new-born child. A 

 " short circuit," however, is established by the ductus venosus, 

 which connects the left branch of the portal vein to the 

 inferior vena cava. In this way most of the pure placental 

 blood passes directly from the umbilical vein to the inferior 

 vena cava, where it becomes mixed with the impure blood 

 returning from the abdomen and lower limbs. 



Thus the purest blood which enters the fcetal heart is 

 poured into the right atrium (auricle) by the inferior vena cava. 

 As it enters the atrium the blood-stream is directed by a fold 

 of endocardium towards the foramen ovale in the inter-atrial 

 septum, through which it passes to reach the left side of the 

 heart. On the other hand, the impure blood carried by the 

 superior vena cava is directed through the right atrio-ventricular 

 orifice into the right ventricle. 



The purest blood which the fcetal heart distributes is pumped 

 out by the left ventricle into the aorta, by which it is conveyed 

 to the heart-muscle, the upper limbs, the head, neck and brain. 



Since the lungs of the fcetus do not function, it is only 

 necessary that they should be provided with sufficient blood 

 for their own nourishment, but, despite this, the main stem of 

 the pulmonary artery is so large that it does not require to 

 dilate when the lungs become expanded by respiration. The 

 surplus blood from the pulmonary artery passes into the aorta 

 through a communication termed the ductus arteriosus. 



