264 ANIMAL BIOLOGY 



blood as it passes from the left ventricle through the aorta. 

 From this large vessel it may flow into any of the branch 

 arteries which supply the arms, legs, liver or any of the 

 numerous organs of the body. As it passes through the 

 fine capillaries it is collected into veins which ultimately 

 lead into the two venae cavae which discharge into the right 

 auricle. From here it passes through a valve into the 

 right ventricle whence it is forced out through another 

 valve into the trunk that supplies the pulmonary arteries 

 leading to the lungs. After passing through the capil- 

 laries of the lungs, it is returned by the pulmonary veins 

 to the left auricle, whence it flows through a valve into 

 the left ventricle, thus completing the entire circuit of 

 both the pulmonary and the systemic circulations. 



The vessels which carry the blood to and from the organs 

 of the body fall into three classes, the arteries, carrying the 

 blood from the heart; the "veins which return the blood to 

 the heart; and the minute capillaries which connect the 

 arteries with the veins. Both arteries and veins have 

 muscular walls, but the walls are thicker in the arteries in 

 adaptation to the greater pressure to which they are 

 subjected by the pumping action of the heart. As the 

 arterial walls are elastic they expand somewhat as blood 

 is forced into them by each contraction of the ventricles. 

 This periodic expansion forms the pulse which can plainly 

 be felt in the arteries of the wrist and neck. The rapidity 

 of the pulse, normally about 75 beats per minute, is an 

 index of the activity of the heart. As one may readily 

 demonstrate upon himself, the pulse is quickened by exer- 

 cise and excitement; it is also quickened during a fever, 

 its rate often affording the physician a clue to the condi- 

 tion of the patient. In other conditions of illness the 

 pulse may be unusually slow and weak. 



As the arteries branch into smaller and smaller vessels 



