212 THE CIRCULATORY SYSTEM [CH. XVIII. 



around the auriculo-ventricular opening, and is fixed to a tendinous 

 ring which encircles the orifice between the auricle and ventricle, 

 and receives the insertions of the muscular fibres of both. In each 

 principal cusp may be distinguished a central part, extending from 

 base to apex, and including about half its width. It is thicker and 

 much tougher than the border pieces or edges. 



While the bases of the cusps of the valves are fixed to the tendinous 

 rings, their ventricular surface and borders are fastened by slender ten- 

 dinous fibres, the chordce tendinece, to the internal surface of the walls 

 of the ventricles, the muscular fibres of which project into the 

 ventricular cavity in the form of bundles or columns the columnce 

 carnece. These columns are not all alike, for while some are attached 

 along their whole length on one side, and by their extremities, others 

 are attached only by their extremities ; and a third set, to which the 

 name musculi papillares has been given, are attached to the wall of 

 the ventricle by one extremity only, the other projecting, papilla- 

 like, into the cavity of the ventricle (5, fig. 218), and having attached 

 to it chordae tendineae. Of the tendinous cords, besides those which 

 pass to the margins of the valves, there are some of especial strength, 

 which pass to the edges of the middle and thicker portions of the 

 cusps before referred to. The ends of these cords are spread out in 

 the substance of the valve, giving its middle piece its peculiar strength 

 and toughness ; and from the sides numerous other more slender 

 and branching cords are given off, which are attached all over the 

 ventricular surface of the adjacent border-pieces of the principal 

 portions of the valves, as well as to those smaller portions which 

 have been mentioned as lying one between each two principal ones. 

 Moreover, the musculi papillares are so placed that, from the 

 summit of each, tendinous cords proceed to the adjacent halves of 

 two of the principal divisions, and to one intermediate or smaller 

 division, of the valve. 



The preceding description applies equally to the mitral and 

 tricuspid valve ; but it should be added that the mitral is considerably 

 thicker and stronger than the tricuspid, in accordance with the 

 greater force which it is called upon to resist. 



The semilunar valves guard the orifices of the pulmonary artery 

 and of the aorta. They are nearly alike on both sides of the heart ; 

 but the aortic valves are altogether thicker and more strongly con- 

 structed than the pulmonary valves, in accordance with the greater 

 pressure which they have to withstand. Each valve consists of three 

 parts which are of semilunar shape, the convex margin of each being 

 attached to a fibrous ring at the place of junction of the artery to 

 the ventricle, and the concave or nearly straight border being free, 

 so as to form a little pouch like a watch-pocket (7, fig. 218). In 

 the centre of the free edge of the pouch, which contains a fine cord 



