THORACIC CAVITY 73 



surface ; others are attached to the wall at each extremity, 

 but are free in the rest of their extent. The cavity of the 

 ventricle is invaded, however, not only by the trabeculae 

 carneae, but also by a number of conical muscular projections, 

 the musculi papillares. These are attached by their bases to 

 the wall of the ventricle, whilst their apices are connected, 

 by a number of tendinous strands, to the margins and the 

 ventricular surfaces of the cusps of the atrio-ventricular valve. 

 As a rule there is one large anterior papillary muscle attached 

 to the anterior wall, a large inferior papillary muscle attached 

 to the inferior wall, and a number of smaller papillary muscles 

 attached to the septal wall. Occasionally the anterior and 

 inferior muscles are represented by a number of smaller 

 projections. It must be noted that the chordae tendineae 

 from each papillary muscle, or group of papillary muscles, 

 gain insertion into the margins and ventricular surfaces of 

 two adjacent cusps of the valve. The result of this arrange- 

 ment is, as the papillary muscles contract simultaneously with 

 the contraction of the general wall of the ventricle, that the 

 chordae tendineae hold the margins of the cusps together and 

 prevent them being driven backwards into the atrium. 



One of the trabeculae carneae, which is usually strong and 

 well marked, passes across the cavity from the septum to the 

 base of the anterior papillary muscle. This is the moderator 

 band. It tends to prevent over-distension of the cavity of 

 the ventricle, by fixing the more yielding anterior wall of the 

 ventricle to the more solid septum. 



There is one opening of entrance into the right ventricle, 

 the atrio-ventricular, and one opening of exit, the pulmonary 

 orifice. Each is guarded by a valve. 



The right atrio-ventricular orifice lies at the lower and 

 posterior part of the right ventricle, its centre being behind 

 the middle of the sternum at the level of the fourth intercostal 

 space. It is about one inch in diameter, and is surrounded 

 by a fibrous ring. It admits the tips of three fingers, 

 and it is guarded by a valve possessing three cusps, an 

 anterior, a medial, and an inferior. The anterior cusp 

 intervenes between the atrio-ventricular orifice and the conus 

 arteriosus. The medial cusp lies in relation with the septal 

 wall ; and the inferior cusp with the inferior wall of the 

 ventricle. 



The bases of the cusps are attached to the fibrous ring 



