58 



ZOOLOGY. 



99. Passage of the Blood into the Cavities of the 

 Heart. This has been already de- 

 a scribed, but may be thus again briefly 



adverted to. The blood having an 

 arterial character, returns from the 

 lungs into the left auricle. This 

 auricle communicates with the left 

 ventricle by an opening called the left 

 auriculo-ventricular aperture, through 

 which the blood flows from the auricle 

 into the ventricle. In this passage 

 is placed a valve, called mitral, permit- 

 ting the blood to pass, but not to re- 

 turn. The arterial blood now collected 

 in the ventricle is acted on by this fleshy cavity, and forced 

 into the aorta ; at the entrance of the aorta are three valves, 



Fig. 43. Section of the 

 Heart.* 



Fig. 44. Valvules of the Heart and Arteries.t 



* Theoretic section of the heart, to show the mechanism of the play of the 

 valves : a, auricle receiving the veins e e ; b, ventricle separated from the 

 auricle by the valvules c ; d, fleshy bridles or stays of these valvules ; 

 -f, artery springing from the ventricle j g, valvules situated at the entrance 

 of the artery. 



t 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. 



