3i8 THE ANIMALS AND MAN 



upon it. A short rest gives the blood time to carry away the 

 wastes from the tissues. 



CIRCULATION 



In order that the blood may find its way from the absorp- 

 tive cells of the intestine to every tissue of the body and 

 from these to the lungs, it must circulate in a continuous 

 flow. This it does by means of a closed system of vessels, 

 the blood-vessels, and a pumping organ, the heart. The 

 arteries carry blood away from the heart to the system and 

 the lungs. The veins carry blood to the heart from the sys- 

 tem and the lungs. Tiny anastomosing (many branched) 

 vessels called capillaries connect the arteries and veins. 



Structure and position of the heart. — In fig. 159 the 

 heart may be seen lying in its natural position, a little to the 

 left of the thoracic cavity, just above the diaphragm and be- 

 tween the two lobes of the lungs. The apex points down- 

 ward and to the left. 



The heart is enclosed within a sac called the pericar- 

 dium. The walls of the heart are thick and muscular 

 and are supplied with blood-vessels and nerves. 



Internal structure of the heart. — The heart is divided 

 by a muscular partition into right and left halves, having 

 no' intercommunication. Each half is separated into an 

 upper and lower cavity, the auricle and ventricle. The 

 right auricle opens into the right ventricle and the left aur- 

 icle opens into the left ventricle. The blood flows from the 

 auricles into the ventricles and out into the arteries. Fig. 

 162 shows the left side of the heart opened so that the inside 

 may be seen. The opening between the left auricle and the 

 left ventricle is guarded by a valve, the mitral valve, with 

 two flaps (6,6'). These flaps are fastened by strong cords 

 to tiny muscular pillars on the walls of the ventriculus (5,5 ). 

 The cords prevent the flaps from being forced backward 

 into the auricle when the heart beats. 



