THE VASCULAR MECHANISM. '',I79 



of flattened cells united by cement substance. Their calibre varies 

 slightly, but the average diameter is little wider than that of a red 

 corpuscle. 



The veins also possess three coats, but their walls are thinner and 

 contain much less muscular and elastic tissue than the arteries, and 

 are strengthened by the presence of a considerable amount of fibrous 

 tissue, especially in the outer coat. Many veins have .valves consisting 

 of fibrous tissue covered on each surface by endothelial cells, and so 

 arranged that they allow blood to flow towards the heart, but prevent 

 any flow in the opposite direction. 



The arteries remain patent when divided, and a high internal 

 pressure is required to distend their thick muscular and elastic walls, 

 whereas the thin-walled veins collapse when opened, and become 

 distended under a very low pressure. 



THE COURSE OF THE CIRCULATION. 



The heart beats rhythmically from seventy to seventy-two times a 

 minute. Tfe6^bfiaJLConsis_ts_ of. the c^ntractioiLaiLJJh.e_aiiiieles v followed 

 almost immediately by that of the ventricles, and is succeeded by a pause, 

 during which the whole heart is completely relaxed. The contraction of 

 the auricles and ventricles is spoken of as auricular or ventricular systole, 

 the period during which the heart is relaxed being called the diastole. 

 At each beat the ventricles expel blood into the aorta and pulmonary 

 artery, from which it is distributed by the former to the body as a whole 

 and by the latter to the lungs. 



The blood entering the aorta from the left ventricle is conveyed by 

 the arteries arising from it to the capillaries of the various organs of 

 the body, with the exception of the lungs. From these organs it is 

 returned by veins, which unite with each other, eventually forming the 

 vense cavse, which open into the right auricle. The blood passes from 

 the right auricle into the. right ventricle, from which it is forced into 

 the pulmonary artery ; it then flows along the subdivisions of this 

 artery through the pulmonary capillaries into the pulmonary veins 

 (two for each lung), and thence into the left auricle. From the left 

 auricle the blood enters the left ventricle and is again sent out into the 

 aorta (fig. 54). 



In the abdomen, the blood passes through a double set of capillaries. 

 The veins which receive blood from the digestive tract and spleen unite 

 to form a single large vein, the portal vein, which on reaching the liver 

 again breaks up into capillaries ; these open into the hepatic veins 

 which join the inferior vena cava. 



