THE RIGHT VENTRICLE. 339 



ings, the tricuspid and pulmonary ; two valves, the 

 tricuspid and semilunar; the columnae carneae and 

 chorda? tendineae. 



The tricuspid opening has been described. 



The pulmonary opening is the constricted orifice at 

 the summit of the right conus arteriosus. It will admit 

 the thumb, and measures accurately three and one-half 

 inches in circumference. 



The tricuspid valves consist of three leaflets, the 

 inner, between the pulmonary orifice and tricuspid open- 

 ing ; the anterior, and posterior. Their edges are joined 

 together at their attachment to the tricuspid fibrous ring. 

 They are formed by a reduplication of the endocardium 

 and re-inforced by bundles of fibrous tissue. Each leaflet 

 is triangular in form and is attached by its free end to 

 the chordae tendineae, which are inserted at three differ- 

 ent places on the valve, first, on the central thickened 

 part of the valve; second, on the free margin of the 

 valve; third, on the valve at its insertion into the tri- 

 cuspid fibrous ring. 



The semilunar valves are three semilunar folds or 

 festoons of the endocardium, placed at the beginning of 

 the pulmonary artery ; they open into the vessel, have 

 thickened edges, and present at their middle a fibro- 

 cartilaginous nodule, the corpus Arantii, which serves to 

 close the valve perfectly, thus preventing regurgitation. 



The columnce carnece are bundles of muscular fibres 

 disposed in three different ways. Some are mere ridges 

 on the inner surface of the ventricle, others are attached 

 .by their two extremities, and the third set are attached 

 by one extremity to the ventricle ; the other is con- 

 tinued as a tendinous cord (the chordae tendineae) to 

 the valve. 



The left auricle is smaller than the right; its walls 



