CHAP. XL] THE VASCULAR SYSTEM. 137 



General summary of the circulation. The perfect circulation 

 of the blood is dependent upon certain factors, the chief of 

 which are : (1) the character of the heart-beat ; (2) the con- 

 traction and relaxation of the minute arteries: (3) the elas- 

 ticity and extensibility of the arterial walls ; (4) the perfect 

 adjustment of the valves. 



The character of the heart-beat is mainly determined by the 

 condition of its muscular substance, and any interference with 

 the nutrition of the heart leading to degeneration of its mus- 

 cular walls very seriously affects the heart's action. 



The contraction and relaxation of the smaller arteries is under 

 the influence of the nervous system. The muscular tissue found 

 in the walls of these vessels is supplied with non-medullated 

 nerve-fibres. Stimulation of one set of these fibres (vaso- 

 constrictor) causes contraction of the muscle-fibres and con- 

 striction of the arteries ; stimulation of a second set (vaso- 

 dilator) causes a relaxation of the muscle-fibres, and dilatation of 

 the arteries. The widening and narrowing of these arteries not 

 only affects 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 stiff er and more rigid, and 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. 



FCETAL CIRCULATION. The peculiarities of the foetal cir- 

 culation, 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 



