THE HEAET AND CIRCULATION OF BLOOD. 313 



from walls of the heart. In the right ventricle we find a vessel, 

 the pulmonary artery (^Pu.A) ; in the left auricle the pulmonary 

 veins (Pu. V) ; and in the left ventricle the opening of the great 

 aorta {A.ao and A.d). 



Let us now trace the course of the blood during circulation. 

 The right auricle receives the venous hlood from the head, neck, 

 and anterior extremities by the superior vena cava ; it also 

 receives venous blood from the lower parts of the body by the 

 inferior vena cava. The right auricle then, holding venous 

 blood, contracts, and by so doing expels the blood through the 

 tricuspid valves into the right ventricle. The right ventricle 

 then empties itself by contracting, the blood being forced 

 through the semi-lunar valves into the pulmonary artery, which 

 carries the venous blood to the lungs ; here the artery splits up 

 into a number of minute tubes, the capillaries, composed of 

 single-celled walls, thus bringing the red blood-corpuscles in 

 close contact with the air. Another set of capillaries also arise 

 in the lungs ; these unite after collecting the blood, and form 

 several pulmonary veins, which carry the blood back to the left 

 'auricle. During the passage of the blood through the lungs the 

 venous blood gives up its CO2, which passes through into the 

 pulmonary alveoli, and so out by the respiratory tubes ■ at the 

 same time it takes up the oxygen in the inspired air. The 

 venous blood of the pulmonary artery thus becomes converted 

 into arterial blood, carried back by the pulmonary veins to the 

 left auricle. The left auricle when fuU contracts and drives the 

 arterial blood through the opening and mitral valves into the 

 left ventricle. From the ventricle the arterial blood is pumped 

 into the aorta, which sends one branch to the anterior regions 

 and one to the posterior, carrying the pure blood to the organs 

 and system generally. In the organs, &c., these two large 

 arteries with their minor divisions break up into capillaries, the 

 blood being collected again by other capillaries which unite with 

 the superior and inferior venss cavse, and so back to the right 

 auricle, from which we started, the course of the circulation 



