04 ZOOLOGY. 



into the aorta; at the entrance of the aorta are three valve-. 

 the semilunar, permitting the blood to flow freely into the artery 

 but not to return into the ventricle; distributed by the aorta 

 and its branches to every part of the body and to the fleshy 

 walls of the heart itself, the blood is taken up by the venous 

 capillaries, and returned to the right auricle by the two venae 

 cavse and coronary vein ; collected in this auricle it is strictly 

 venous blood, but has already received the product of diges- 

 tion. From this auricle, and by its action, the blood is driven 

 through the right auriculo-ventricular orifice into the right 

 ventricle; the tricuspid valve permits it to pass into the 

 ventricle, but not to return. By the action of this ventricle 

 the blood is forced into the pulmonary artery, and by it to 

 the lungs, thence to be returned to the left auricle (aerated) 

 by means of the pulmonary veins. At the entrance of the 

 pulmonary artery there are, as in the aorta, three semilunar 

 valves, permitting the blood to pass freely into the artery, 

 but not to return. From the mechanism alone of the valves 

 alluded to, the circulation of the blood might easily have 

 been foretold. 



Fig. 35. Valvules of the Heart and Arteries.* 



* Upper surface of the heart, the auricles having been removed. 

 1, auriculo-ventricular orifice, obliterated by the tricuspid valve ; 2, fibrous 

 ring surrounding the orifice ; 3, left auriculo-ventricular orifice, surrounded 

 by a ring, and closed by the mitral valve ; 4, orifice leading into the aorta 

 from the left ventricle, closed by the semilunar valves ; 5, orifice leading 

 into the pulmonary artery from the right ventricle, also provided with three 

 semilunar valves. 



