THE CIRCULATION OF THE BLOOD. 179 



ing portions, so as to form an annular membrane 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 distin- 

 guished 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 chord tendinew, 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 papil- 

 lares 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 (4, fig. 148), and having attached to it chordae tendineae. Of 

 the tendinous cords, besides those which pass from the walls of the 

 ventricle and the musculi papillares to the margins of the valves, there 

 are some of especial strength, which pass from the same parts 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 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 tricus- 

 pid 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 constructed 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 



