Circulation, Assimilation and Excretion 



the force. Like a pump, the heart has valves mitral valves, 

 tricuspid valves and semilunar valves. These valves are thin, 

 strong doors between the auricles and ventricles. They readily 

 open to allow the passage of blood forward, but immediately close 

 after the blood passes, and are held down tightly by its back- 

 ward pressure, so that it does not return but flows onward. When 

 the mitral valve, which is composed of two flaps, opens, the blood 

 flows from the left auricle to the left ventricle. When the 

 tricuspid valve, which is composed of three flaps, opens, the blood 

 flows from the right auricle into the right ventricle. Other valves 

 are found at the openings of the arteries leading from the ventricles. 

 They are made up of three flaps and on account of the resemblance 

 of each flap to a half -moon they are called semilunar valves. 



The Arteries. The vessels in which the blood flows from 

 the heart to the various organs of the body are called arteries. 

 Arteries are well adapted by structure to do their work. They 

 are pipe-like in their make-up and are covered with a layer of 

 connective tissue that protects the inner lining which is composed 

 of cells. They are elastic and for the most part do not lie near the 

 surface of the body. 



The Pulse. The blood flows through the arteries with a 

 wave-like motion. This motion causes a throb which may be felt 

 wherever an artery passes near the surface of the body, as in the 

 wrist. This throb is called the pulse. The number of heart beats 

 per minute may be determined by counting the throbs for that 

 length of time. In an adult the normal pulse varies from seventy 

 to eighty throbs per minute. 



The Veins. The blood returns from the organs of the body 

 to the heart through a series of pipe-like vessels called veins, 

 formed by the coming together of a network of capillaries that 

 originates at the ends of the arteries. The veins have thinner and 

 less strong walls than the arteries and the blood current moves 

 more slowly. They have valves at intervals which prevent a back- 

 ward flow but do not hinder the onward course of the blood to the 

 heart. 



The Capillaries. The capillaries are extensions of the 

 arteries that develop from the lining of the arteries and lead to 



