CIRCULATION OF THE BLOOD. 6l 



All the tissues of the body may be regarded as so many 

 islands lying between the capillary streams. 



The Blood Flow in the Veins. When the blood re- 

 collects in the veins it is entering narrower channels, and 

 its rate is quickened ; but as the veins are wider than the 

 arteries, the stream does not enter the heart with the veloc- 

 ity with which it left that organ. The veins hold more 

 blood than the arteries, and in dissecting the cat or rabbit 

 it will be noticed that the arteries are emptied of blood; 

 that the tissues of most of the organs are fairly free from 

 blood ; but -that the great veins, such as the caval veins, 

 are full. 



Blood Tubes compared to Two Funnels. If the blood 

 tubes leaving the heart could all be united, they would be 

 best represented by a funnel with its tube connected with 

 the heart. If another funnel were placed with its mouth 

 to the mouth of the first, their point of union, the widest 

 point, would represent the capillaries; and if the second 

 funnel had a wider tube than the first, it would fairly rep- 

 resent the veins which return the blood to the heart. 



Nourishment of the Walls of the Heart and Blood 

 Tubes. The cardiac (coronary) arteries spring from the 

 aorta just above the semilunar valves, and send blood into 

 the muscular walls of the heart ; and these arteries, like 

 others, divide, forming capillaries, through which the heart 

 muscle is nourished, The cardiac veins return the blood 

 to the right auricle. 



Influence of Gravity on Circulation. Although the 

 heart pumps the blood around through the body inde- 

 pendent of the force of gravity, yet the circulation is influ- 

 enced by this force. For instance, a person who has 



