CIRCULATION 



45 



The opening between the right auricle and the right ventricle 

 is guarded by the right auriculo-ventricular (tricuspid) valve; 

 the opening between the right ventricle and the pulmonary ar- 

 tery is guarded by the right semilunar valve. 



The opening between the left ventricle and the left auricle is 

 guarded by the left auriculo-ventricular (bicuspid) valve, and 

 the opening to the aorta from the left ventricle is guarded by 



Fig. 25. — Capillary Circulation. (Eddy.) 



In the web of a frog's foot (X 100) ; a, 6, small veins; d, capillaries in 

 which the corpuscles are seen to follow one another in single series; c, 

 pigment cells in the skin. 



the left semilunar valve. Valves at these places are necessary to 

 prevent the blood from moving backward instead of forward. 

 Course of the blood. — Entering the right auricle of the heart 

 through two large veins, the anterior and posterior vena cava?, 

 and the coronary vein, the blood passes through this auricle 

 into the right ventricle, thence to the lungs, through the pul- 

 monary artery back to the left auricle, through four to eight 

 pulmonary veins, past the valves on the left side, and finally to 

 the left ventricle which sends it through the systemic circuit. 

 The systemic blood leaves the left ventricle through a large 

 artery, the aorta, which distributes blood through its branches to 

 all parts of the body. 



