THE LEFT VENTRICLE 



533 



The Left Ventricle 



The left ventricle (Ventriculus sinister) forms the left posterior part of the 

 ventricular mass. It is more regularly conical than the right ventricle and its 

 wall is much thicker except at the apex. It forms all of the posterior border of 

 the ventricular part and the apex of the heart. Its l)ase is largely continuous with 

 the left atrium, with which it communicates through the left atrio-ventricular 

 opening, but in front it opens into the aorta. The cavity usually appears smaller 

 than that of the right ventricle in the dead subject, on account of the greater con- 

 traction of its wall. It is almost circular in cross-section. 



The left atrio-ventricular orifice (Ostium venosum sinistrum) is opposite to 

 the fifth rib and intercostal space. It is almost circular and is guarded b}' the 



Origin of right coronary artery 



Conus arteriosus 



Pulmonary openiiuj 



Origin of left coronary 

 artery 



Rigid alrio-ven- 

 tricular opening 



Aortic opening 



Left atrio-ventricular 

 opening. 



Fig. 425. — The B.a.ses of the Ventricles of the Heart of the Horse. 

 The atria have been removed and the aorta and pulmonary artery cut off short. The left ventricle is contracted. 



p.r.. Pulmonary valve; a.v., aortic valve. 



bicuspid or mitral valve (\^alvula l^icuspidalis). The two segments of this valve 

 are larger and thicker than those of the right side of the heart. One is placed in 

 front and separates the atrio-ventricular and aortic openings. The other is placed 

 behind and laterally and is usually divided into two or three flaps. 



The aortic orifice (Ostium aorticum) is directed upward and slightly forward. 

 It is situated opposite to the fourth rib. It is guarded by the aortic valve, composed 

 of three semilunar cusps (Valvulse semilunares aortse) ; one cusp is anterior, the 

 others right and left posterior. They are similar to those of the pulmonary valve, 

 but are much stronger. The free edge of each contains a central nodule of fibrous 

 tissue, the corpus Arantii (Nodulus valvulse semilunaris). 



The chordae tendineae are fewer but larger than those of the right ven- 

 tricle. 



There are two large musculi papillares, one on each side; they are usually 



