ioo THORAX 



sented 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 the arrangement 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 at the same time 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. It is called 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 which possesses three cusps, and 

 is called, therefore, the tricuspid valve. 



Valvula Tricuspidalis. The three cusps of the tricuspid 

 valve occupy definite positions : one is anterior, another 

 medial, and the third 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. 

 Occasionally small additional cusps are interposed between 

 the three main cusps. 



The bases of the cusps are attached to the fibrous ring 

 round the margin of the orifice. Their apices, margins 

 and ventricular surfaces are attached to the chordae tendineae. 

 Their atrial surfaces, over which blood flows as it enters 

 the ventricle, are stnao,th,' and their ventricular surfaces are 

 more or less toughened- by trie-, attachment of the chordae 

 tendineae, but the roughening is less marked on the ventricular 

 surface of thfe anterior- cusp over winch the blood- ows as it 

 passes through the c6nus* arteriosus to the puln?orjiary orifice. 



