THE RIGHT VENTRICLE. 401 



named muscnli papillares, which give attachment to the little 

 tendinous cords of the valve of the auriculo- ventricular opening. 



The auric u.lo-ventricular orifice is situate in the base of the Opening 

 ventricle, and behind the right half of the sternum, on a level a u r ie * 

 with the fourth intercostal space. It is oval in shape, and position, 

 measures about four inches in circumference, being slightly larger ^. audl 

 than the corresponding aperture of the left side. 



Fixed around the opening is a large membranous valve, which is guarded 

 projects into the cavity of the ventricle. At its attached margin the cuspid 

 valve is undivided : but its lower part is notched, so as to form valve > 

 three pendent cusps or tongues, whence the name tricuspid is given 

 to it. Into the cusps are inserted small fibrous bands the chordae Cusps: 

 tendinoe, which unite them to the muscular wall of the ventricle. 

 The three cusps are thus placed ; one (marginal) is against the marginal ; 

 anterior wall of the ventricle ; posteriorly, another (septal) touches septal; 

 the septum ; and the third (infundibular] , the largest and most iufundi- 

 moveable, is placed to the left, between the auriculo-ventricular bular ' 

 opening and the infundibulum. 



The tricuspid valve consists of a duplicature of the lining mem- Structure of 

 brane of the heart, enclosing fibrous tissue. The central part of ^ 

 each tongue is strong, while the edges are thin and notched ; and 

 between the main pieces there are often thinner intermediate points. 



The chorda tendince, which keep the valve in place, ascend from attachment 

 the musculi papillares in the intervals between the cusps, and are 

 connected in each space with the two pieces of the valve bounding 

 it. They end on the surface of the cusps turned away from 

 the opening, a few reaching the attached upper margin ; but the 

 greater number join the central thickened part, and the thin edge 

 and point of the cusp. 



The papillary muscles are collected into two principal groups, papillary 

 i j- ., , i j j-i i muscles, 



an anterior sending its tendons to the marginal and infundibular 



cusps, and a posterior, to the marginal and septal cusps. In the 

 interval between the infundibular and septal segments of the valve 

 the tendinous cords are small, and spring from the septum. 



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

 the opening into the auricle ; and its protrusion into the latter cavity 

 during the contraction of the ventricle is arrested by the small 

 tendinous cords. The closure of this valve assists in producing the 

 first sound of the heart. 



The mouth of the pulmonary artery will be seen when the Pulmonary 

 incision in the anterior wall of the ventricle is prolonged into it. orlfice : 

 The opening is circular, with a diameter of about an inch. It sizeand 

 occupies the summit of the funnel-shaped portion of the ventricle, pos! 

 and is placed opposite the upper edge of the third costal cartilage 

 of the left side, close to its junction with the sternum. 



Pulmonary valve. Guarding the orifice of the pulmonary artery its valve of 

 is a valve consisting of three semilunar or sigmoid flaps ; a right threefla P s : 

 and left anterior, and a posterior. Each flap is attached to the side 

 of the vessel by its convex border, and is free at the opposite edge, 

 in the centre of which there is a slightly thickened nodule the 



