282 CIRCULATION OF THE BLOOD IN THE VESSELS. 



situated at the origin of the large arteries, and causes 

 these to flap towards one another, and close the 

 arterial aperture, the recoil force of the elastic walls 

 of the vessel is spent entirely in urging the blood 

 towards .the capillaries. This is the explanation of 

 the fact well-known of the continuous flow of the 

 blood in the capillary vessels, as proved by the unin- 

 terrupted oozing which occurs when the skin in any 

 part of the body is cut or pricked, while in the small 

 arteries it flows by successive jets corresponding to 

 the beats of the heart, as is shown when an artery of 

 a living animal is cut open. 



The blood having traversed the capillary vessels 

 passes into the minute veins which at length unite 

 to form the two great veins of the body (venae cavae) 

 which open into the right or venous auricle of 

 the heart. From the right auricle the venous blood 

 passes into the right ventricle, which drives it into 

 the pulmonary artery and the capillaries of the lung. 

 After it has been aerated by traversing the capillaries 

 spread out upon the pulmonary air-cells, it is collected 

 by pulmonary veins which open into the left auricle 

 of the heart. From the left auricle the blood passes 

 into the left ventricle, by which it is pumped into the 

 aorta and the arteries of the body which come off 

 from this large primitive trunk. 



Derangement of any part of the important apparatus 

 concerned in the circulation of the blood may indi- 

 rectly give rise to disease either by alteration induced 

 in the composition or distribution of the nutrient fluid 

 or in some other manner. But of all the morbid 

 affections of the organs of circulation those which 

 affect the most minute arteries, the capillaries and 

 smaller veins are the most common, most obscure, 

 but by no means the least serious. For although life 

 is not often suddenly cut short by minute structural 

 changes occurring in these vessels, death is none the 

 less certain to take place. Structural and too often 



