CHAP. X.] THE VASCULAR SYSTEM. 115 



the local circulation in different parts of the body, but the 

 amount of resistance they oppose to the arterial impulse also 

 influences in some degree the character of the heart-beat. 



The elasticity and extensibility of the arteries change with 

 the age of the individual. As we grow older the arterial walls 

 grow stiffer and more rigid, a'nd become less well adapted for 

 the unceasing work they are called upon to perform. The 

 valves also show signs of age as years advance, and even if not 

 injured by disease, do not adjust themselves so perfectly as in 

 early life. 



Still, the heart has a marvellous facility for adjusting itself 

 to changed conditions, and the circulation of the blood may 

 go on for years with the integrity of the vascular mechanism 

 greatly impaired. 



FOETAL CIRCULATION. The peculiarities of the foetal circu- 

 lation, leaving details aside, are the direct communication 

 between the two auricles of the heart through an opening 

 called the foramen ovale ; the communication between the pul- 

 monary artery and descending portion of the arch of the aorta 

 by means of a tube called the ductus arteriosus ; and the com- 

 munication between the placenta and the foetus by means of 

 the umbilical cord. 



The arterial blood for the nutrition of the foetus is carried from 

 the placenta along the umbilical cord by the umbilical vein. 

 Entering the foetus at the umbilicus the blood passes upwards 

 to the liver and is conveyed into the inferior vena cava in two 

 different ways. The larger quantity first enters the liver, and 

 alone, or in conjunction with the blood from the portal vein, 

 ramifies through the liver before entering the inferior vena 

 cava, by means of the hepatic veins. The smaller quantity of 

 blood passes directly from the umbilical vein into the inferior 

 vena cava, by a tube called the ductus venosus. 



In the inferior vena cava the blood from the placenta 

 becomes mixed with the blood returning from the lower ex- 

 tremities of the foetus. It enters the right auricle and guided 

 by a valve, the Eustachian valve, passes through the foramen 

 ovale into the left auricle. In the left auricle it unites with a 

 small quantity of blood returned from the lungs by the pul- 

 monary veins. From the left auricle the blood passes into the 

 left ventricle, and is distributed by the aorta almost entirely to 



