THE VASCULAR SYSTEM 389 



special papillary muscles, anterior and posterior, in each ventricle. The 

 right valve is partially divided into three ''cusps" and the left into two. 

 Hence they are known respectively as tricuspid and bicuspid or mitral 



valves. 



At the opening of the aorta and of the pulmonary artery crescentic 

 semilunar valves prevent the return of blood into the ventricles. Each 

 artery contains three of these valves so arranged that under pressure of 

 the blood they meet together and occlude the lumen completely. Near 

 the semilunar valves lie the openings of the coronary arteries which 

 supply blood to the wall of the heart. This blood is returned to the 

 coronary sinus of the right atrium by coronary veins which parallel the 

 coronary arteries. 



Course of Blood in the Heart. The blood from the systemic veins, 

 the precava and the postcava, enters the right atrium and by the con- 

 traction of the atrium is pumped through the tricuspid valve into the 

 right ventricle. From the right ventricle the blood is carried by the 

 pulmonary artery to the lungs, returning from the lungs by the four 

 pulmonary veins to the left atrium. Forced by the contraction of the 

 atrium through the bicuspid valve into the left ventricle it is pumped into 

 the aorta and to all parts of the body. 



PULMONARY CIRCULATION 



The pulmonary artery carries impure or venous blood from the right 

 ventricle to the lungs. Near the heart it divides into right and left 

 branches which connect with the corresponding lungs. At the point of 

 separation of right and left pulmonary arteries the ligamentum arteriosum 

 connects them with the aorta. 



From the lungs, blood is returned to the left atrium by pulmonary 

 veins which, unlike other veins, convey aerated blood. While there are 

 usually four pulmonary veins, occasionally there are five and the blood 

 from the middle right lobe enters the atrium independently. Within 

 the lung lobes the pulmonary veins parallel the arteries. 



Arteries. Arteries convey blood away from the heart, and because 

 they are subjected to considerable pressure when the heart contracts, 

 their walls are correspondingly thick and elastic. A cross section shows 

 three layers, an intima, a relatively thin layer consisting of the lining 

 endothelium and a connective tissue layer with elastic fibers, a media, a 

 relatively thick layer of muscle and elastic fibers, and an externa, a. 

 layer of loose connective tissue consisting largely of white inelastic fibers. 

 Because of the strength and elasticity of their walls, arteries usually do not 

 collapse after death. (Fig. 300) 



Arteries have their own circulatory and nervous supply. The capil- 

 laries of the walls are the vasa vasorum. The nerves are branches of the 



