\ 



516 THE BLOOD-VASCULAR SYSTEM 



near the attached margin. The valves are smooth and glistening on the atrial 

 aspect, but rough and fasciculated, from the attachment of the chordae, on the 

 ventricular. The cusps of the mitral valve are called anterior and posterior; those 

 of the tricuspid, anterior, posterior and medial. Each cusp receives chordae from 

 more than one papillary muscle and each papillary muscle sends chordae to more 

 than one cusp. The chordae tendineae of the mitral valve are thicker than those 

 of the tricuspid (figs. 428, 429). 



VENTRICULAR PORTION 



The ventricles form the greater portion of the heart. In the adult the relation 

 of the ventricles to one another is as follows. The left [ventriculus sinister] 

 has the form of a narrow cone, the apex of which is the apex of the heart. The 

 right ventricle [ventriculus dexter] is crescentic in section and appears to be 

 partially wrapped around the right or lower wall of the left ventricle which forms 

 the septum ventriculorum (fig. 426). The left ventricle forms the margo ob- 

 tusus of the heart, about half the diaphragmatic surface, and a slight part of the 

 sterno-costal surface. The right ventricle forms about half the diaphragmatic 

 surface and the major part of the sterno-costal surface; it takes no share in the 

 formation of the apex of the heart. 



The interventricular septum [septum ventriculorum] is thick and muscular 

 except for a small area near the root of the aorta which is membranous [septum 

 membranaceum ventriculorum]. The latter can be seen from the left ventricle in 

 the angle between the attached edges of the right and posterior aortic valves 

 (fig. 429). The membranous septum is partly concealed from the right heart by 

 the medial cusp of the tricuspid valve which is attached to it near its upper part. 

 The portion of the membranous septum above the medial tricuspid cusp is 

 therefore atrio-ventricular, i. e., between the right atrium and left ventricle. 



The membranous septum is the extreme lower part of the independent septum (s. aorticum) 

 which divides the aortic root from the pulmonary artery and conus arteriosus (and partially 

 subdivides, also, the right ventricle by separating the conus arteriosus from the remainder of 

 the ventricle). The relation of the part of the aortic septum between the conus arteriosus and 

 aortic root to the septum ventriculorum is beautifully shown by His, in fig. 427. 



The greater part of the interior of the ventricles is thrown into ridges by myocardial 

 bundles of large size. These fasciculi [trabeculae cordis] either stand out in rehef only, or, by 

 being undermined, form bands covered except at either end by endothehum. A careful exam- 

 ination of the endocardium of fresh hearts will reveal a plexiform network of Purkinje fibres. 

 These fibres, belonging to the atrio-ventricular conducting system, become very obvious when 

 the endocardium has been exposed to the air long enough to become partially dry. 



The wall of the right ventricle [ventriculus dexter] (figs. 427, 428) is much 

 thicker than that of the atria, but less so than that of the left ventricle. The 

 upper and anterior part of the right ventricle is in relation posteriorly with the root 

 of the aorta. This portion of the ventricle is called the conus arteriosus and is 

 separated from the remainder of the right ventricle by a muscular spur which 

 extends from the back of the conus to the right venous ostium. The spur is the 

 crista supra ventricularis; its relation to the partition between the conus and 

 aorta, and to the septum membranaceum, shows that it is the free edge of the 

 embryonic aortic septum (see morphogenesis of the heart). 



Two papillary muscles in the right ventricle are constant in position, the 

 large anterior papillary muscle, and tlie small papillary muscle of the conus 

 (Luschka). The anterior papillary is situated on the sterno-costal wall, near the 

 junction of this with the septal wall. The papillary of the conus is placed just 

 below the septal end of the crista supraventricularis. The posterior papillary 

 muscles form an irregular group springing from the diaphragmatic wall. Some 

 chordffi tendineae stretch directly from the septal wall (with or without small 

 muscular elevations at their l)asos) to the medial cusp of the tricuspid valve. The 

 chordjn tendin(!ie from the anterior papillary go to the anterior and posterior 

 cusps; those from the conus papillary to the medial and anterior, and those from 

 the posterior papillary muscl(!S to the medial and posterior cusps of the tricuspid 

 valve, respectively. 



There is frequently a band of myocardium extending from the septal wall of the right ven- 

 tricle to the anterior papillary muscle near its middle. This is the moderator band, which 

 contains a part of the right limb of the atrio-ventricular bundle. If the moderator band joins 



