318 DISSECTION OF THE THORAX. 



third set, which are fewer in number and much the largest, project into 

 the cavity, and form rounded bundles, named musculi papillares ; these 

 give attachment by their free ends to the little tendinous cords of the valve 

 of the auriculo-ventricular opening. 



The auriculo-ventricular orifice (c) is situate in the base of the ventri- 

 cle, and is opposite the centre of the sternum, between the third costal carr 

 tilages. It is slightly larger than the corresponding aperture of the left 

 side of the heart. It is oval from side to side, its shape being maintained 

 by a strong fibrous band around it ; and it measures one inch and a quar- 

 ter in diameter. 



Prolonged from the circumference of the opening is a thin membranous 

 valve, which projects into the cavity of the ventricle. Near its attach- 

 ment to the heart the valve is undivided, but it presents three chief points 

 at its lower margin, and is named tricuspid ; to the lower margin are 

 attached small tendinous cords (chordae tendinese), which unite it to the 

 muscular bundles of the ventricle. Its three slips or tongues are thus 

 placed : one touches the front of the ventricle ; another is in contact with 

 the posterior wall ; and the remaining slip, the largest and most movable, 

 is interposed between the aperture into the auricle and pulmonary artery. 



The tricuspid valve is constructed by the lining membrane of the heart, 

 which incloses fibrous tissue. The central part of each tongue is strong, 

 whilst the edges are thin and notched ; and between the primary pieces 

 there are sometimes secondary points (Kiirschner). 



The chordae tendinee which keep the valve in position ascend from the 

 musculi papillares into the intervals between the pieces of the valve, and 

 are connected with both. 1 They end on the surface of the valve turned 

 away from the opening ; some reaching the attached upper margin ; 

 others entering the central thickened part, and the thin edge and point 

 of the tongue. 



As the blood enters the cavity, the valve is raised so as to close the 

 opening into the auricle ; and its protrusion into the other cavity during 

 the contraction of the ventricle is arrested by the small tendinous cords. 

 The valve participates in the production of the first sound of the heart. 



The mouth of the pulmonary artery (g) will be seen when the incision 

 in the anterior wall of the ventricle is prolonged into it. Round in shape, 

 it is situate on the left of the opening into the auricle, and is opposite the 

 inner end of the second intercostal space of the left side. Its diameter is 

 rather less than an inch. Into it the funnel-shaped part of the right ventri- 

 cle is prolonged, and in its interior are three semilunar or sigmoid valves. 



Semilunar valves. Each valve is attached to the side of the vessel by 

 its convex border ; and is free by the opposite edge, in which there is a 

 slightly thickened nodule the corpus Arantii. In the wall of the artery 

 opposite each valve is a slight hollow, the sinus of Valsalva, which is 

 better marked in the aorta. 



The valves resemble the tricuspid in structure, for they are formed of 

 fibrous tissue with a covering of the lining membrane. In each valve the 

 fibres have this arrangement : there is one band along the attached mar- 

 gin ; a second along the free edge, which is connected with the projecting 

 nodule ; and a third set of fibres is directed from the nodule across the 



1 The papillary muscles are collected into two principal groups, whose tendons 

 enter the interval on each- side of the anterior tongue of the valve. In the inter- 

 val between the left and posterior segments of the valve the tendinous cords are 

 very small, and are connected with the septum ventriculorum. 



