CIRCULATION I 1 7 



ber is about that of a red-cell diameter, and which are diffusely distributed 

 to organs and tissues everywhere. 



The capillaries then unite -to form tiny veins which, in turn, join to 

 form larger and larger veins. The veins of the lower portions of the body 

 empty into the inferior vena cava, and the veins of the head and neck are 

 tributaries of the superior vena cava. These two large venous channels 

 empt)^ into the right heart, completing what is known as the systemic 

 circulation. 



From the right heart springs the pulmonary artery, which soon divides 

 into two, one for each lung. Each pulmonary artery divides into smaller 

 and smaller arteries and finally into the lung or pulmonary capillaries, 

 which penetrate all parts of the organ. These, again, collect into larger 

 and larger veins, finally forming the pulmonary veins, which empty into 

 the left heart. This makes up the pulmonary circulation. (The left and 

 the right heart are each divided into two chambers.) 



The atria or auricles (the receiving chambers, one on each side of the 

 heart) have distinctly thinner walls than the ventricles (the pumping 

 chambers, one on each side of the heart). The thicker walls of the 

 ventricles are responsible for almost all the pumping action of the heart. 

 The walls of the left ventricle are much thicker than those of the right. 

 This we relate to the greater work done by the left ventricle. It pumps 

 blood through the entire systemic circuit, while the right ventricle has 

 the easier task of pumping blood only through the lungs, a much shorter 

 distance. 



On each side of the heart is a valve system, betu'een auricle and ventricle, 

 which permits blood to flow only from the auricle to the ventricle, and 

 which is closed automatically by blood starting to move in the reverse 

 direction. These are the right and left auriculoventricular valves. Guard- 

 ing each exit from the ventricles are also valves of a somewhat different 

 construction, called the semilunar valves, from the fact that each is made 

 of three half-moon shaped leaflets. The long margin of each leaflet is at- 

 tacked to the vessel wall, the short margin being free. The aortic semilunar 

 valves are located at the beginning of the aorta. They permit blood to 

 flow from the left ventricle into the aorta, but they are closed by any 

 reflux of blood in the reverse direction. The pulmonary semilunar valves 

 lie at the beginning of the pulmonary artery and prevent backflow of 

 blood from the pulmonary artery into the right ventricle. There are no 

 true anatomical valves at the orifices of the left and right auricles, where 

 the veins empty into the heart. 



The automatic rhythmicity of the heart has received much attention. 

 The heart is not entirely peculiar in this respect, for rhythmic responses 

 are the rule in many organs and systems. The intermittent nature of the 

 breathing movements is a good example, involving rhythmic contractions 

 and relaxations of the muscles of breathing. But this is different from the 



