DISSECTION OP THE BACK AND THORAX. 127 



between different parts of the wall. 3. A third variety are blunt, 

 nipple-shaped prominences called musculi papillares, of which there are 

 commonly three in this cavity, one being placed on the wall and two on 

 the septum. Radiating from each of these is a set of fibrous strings — 

 the chordm tendinem, which are attached by their other ends to the seg- 

 ments of the valve guarding the auriculo-ventricular opening. The 

 right auriculo-ventricular opening is situated at the base of the cavity, 

 and is a very large orifice. It is provided with a valve composed of 

 three main cusps, or segments, and hence named tricuspid. Each of these 

 cusps is triangular in shape, being fixed by its base to the wall of the 

 heart, and having its edges free and directed towards those of the 

 adjacent cusps. When the blood stream is rushing through the opening, 

 the segments of the valve hang down into the ventricle, and have one 

 surface directed towards the blood stream, and the other to the wall of 

 the ventricle. The first of these surfaces is smooth ; the other is rough, 

 and to it and the apex and edges of the cusp, the chordae tendinete are 

 attached. When, during contraction of the ventricle, the blood tends 

 to regurgitate through the opening, the cusps are floated upwards, and, 

 meeting each other, close the orifice. To the efficiency of this action, 

 the chordee tendineae passing from the musculi papillares are essential ; 

 for, being attached to the edges and lower surfaces of the cusps, they 

 prevent the latter from being carried right up into the auricle. The three 

 principal cusps generally alternate around the opening with three of 

 much smaller size. There are three musculi papillares, each with its 

 set of chordae tendineae, and three large cusps ; but it will be observed 

 that one set of the chordae tendineae does not pass entirely to one cusp, 

 but divides itself between two adjacent segments. 



Directions. — A better view of the tricuspid valve may now be obtained 

 by cutting through the auriculo-ventricular ring near the point where 

 the first incision was begun in opening the ventricle, selecting the inter- 

 val between two cusps. When some of the chordae tendineae have been 

 cut, this will enable the wall of the ventricle to be thrown outwards. 



When the ventricle contracts, the blood, prevented by the tricuspid 

 valve from passing back into the auricle, is forced upwards into the left- 

 anterior portion of the ventricle, and leaves the cavity by the pulmonary 

 artery. This portion of the cavity, which leads up to the artery, is termed 

 the comis arteriosus. The orifice of the pulmonary artery is surrounded by 

 a valve composed of three crescentic segments, and hence termed the 

 semilunar valve. The convex border of each segment is fixed to the 

 wall of the artery where it springs from the ventricle. The concave 

 border is free, and shows at its mid point a minute, fibro-cartilaginous 

 thickening — the nodulus or corpus Arantii. On each side of the corpus 

 a small cresentic portion near the free edge of the segment, and dis- 

 tinguished from the rest by its thinness, is termed the lunula. One 



