SEMILUNAR VALVES 



517 



the anterior papillary near its base, as it frequently does, it is difficult to distinguish it from the 

 ordinary trabeculae in this situation. 



The term moderator band was originally applied to this bridge or band of muscle under the 

 impression that it prevented overdistention of the ventricle. Subsequent discovery of the 

 conducting system of the heart makes it plain that there is always a band conducting the right 

 limb of the atrio-ventricular bundle from the septum to the anterior papillary muscle. Whether 

 the band is isolated from the other trabeculae, and therefore readily recognizable, appears 

 to depend somewhat upon the relation of the base of the papillary muscle to the septum 

 ventriculorum. 



The wall of the left ventricle [ventriculus sinister] (figs. 427, 429) is very 

 thick except at the extreme apex, and at the membranous septum. In the left 

 ventricle are two large papillary muscles, generally known as anterior and posterior ; 

 both send chordae tendineae to each cusp of the mitral valve. On the septal wall 

 of the ventricle the left hmb of the atrio-ventricular bundle can usually be seen 

 as a broad, flattened band beneath the endocardium. The band appears just 

 below the septum membranaceum and divides into strands which go to the two 

 papillary muscles. The strands in many places bridge across part of the ventricle 

 to reach the papillary muscles covered only by tubes of endocardium. 



These bridging strands connecting the papillary muscles with the septum ventriculorum, 

 which were formerly called "false chordae tendineae," are exactly comparable to the moderator 

 band of the right ventricle which occasionally consists of atrio-ventricular bundle and endo- 

 cardium only. 



SEMILUNAR VALVES 



The semilunar valves [valvulse semilunares] guard the arterial ostia of the 

 ventricles. The aortic ostium is directed upward and slightly forward and to the 

 right; the pulmonary backward and slightly upward and to the left. Each valve, 

 of which there are three to each ostium, is a pocket-like fold of endocardium 

 strengthened by fibrous tissue (fig. 430). The free edge of each valve is directed 

 away from the ventricle, so that excess of pressure within the great vessels brings 



Fig. 430. — Interior View of the Aortic Semilunar Valves. 



Aortic sinus - L'^ 

 Section of fibrous ring ^}^^ \^^ 



Free edge of valve 



Orifice of right coronary 

 artery 



/ Nodulus Arantii 



^k - Lunula 



Decussation fibrous tissue 

 of valve 



the three valves of either ostium into mutual apposition. In the middle of the 

 free edge of each valve there is a small fibro-cartilaginous nodule; radiating from 

 this toward the entire fundus, and along the extreme free edge of the valve, are 

 fibrous thickenings. On either side of the nodule, between the thicker margin 

 and fundus, the valve is thin over a crescentic area called the lunula. 



The aortic valves are called the right, left, and posterior; the pulmonary valves, 

 the right, left, and anterior.* The aortic semilunar valves are stronger than the 

 pulmonary; opposite them there are three dilatations in the aortic wall, the aortic 



* The BNA names of the aortic and pulmonary valves are not based upon their relative 

 positions in the body. From transverse sections through the thorax (see any good atlas) it 

 may be seen that one aortic valve is anterior, one pulmonary valve posterior, and the other 

 aortic and pulmonary valves are right and left. If the removed heart is held so that the ven- 

 tricles are on the right and left of the septum, respectively, the valves take the positions indi- 

 cated by the BNA. The names given by the BNA to the valves, although conventional (like 

 many other terms of orientation applied to parts of the heart), are convenient, particularly from 

 a developmental standpoint. 



