STRUCTURE OF HEART'S VALVES. 105 



valves of the heart, and the manner in which they perform 

 their function of directing the stream of blood in the 

 course which has been just described. The valve between 

 the right auricle and ventricle is named tricuspid (fig. 34), 

 because it presents three principal cusps or pointed portions, 

 and that between the left auricle and ventricle bicuspid or 

 mitral, because it has two such portions (fig. 35). But in 

 both valves there is between each two principal portions a 

 smaller one ; so that more properly, the tricuspid may be 

 described as consisting of six, and the mitral of four, por- 

 tions. Each portion is of triangular form, its apex and 

 sides lying free in the cavity of the ventricle, and its base, 

 which is continuous with the bases of the neighbouring 

 portions, so as to form an annular membrane around the 

 auriculo-ventricular opening, being fixe'd 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 portion of the valve may be 

 distinguished a middle-piece, extending from its base to 

 its apex, and including about half its width; this piece 

 is thicker, and much tougher and tighter than the border- 

 pieces which are attached loose and flapping at its sides. 



While the bases of the several portions of the valves 

 are fixed to the tendinous rings, their ventricular surfaces 



vein ; + , + , placed in the aarictdo- ventricular groove, where a narrow 

 portion of the adjacent walls of the auricle and ventricle has been pre- 

 served ; 4, 4, cavity of the right ventricle, the upper figure is imme- 

 diately below the semiltinar valves ; 4', large columna carnea or mus- 

 culus papillaris ; 5, 5', 5'', tricuspid valve ; 6, placed in the interior of 

 the pulmonary artery, a part of the anterior wall of that vessel having 

 been removed, and a narrow portion of it preserved at its commence- 

 ment where the semilunar valves are attached ; 7, concavity of the aortic 

 arch close to the cord of the ductus arteriosus ; 8, ascending part or 

 sinus of the arch covered at its commencement by the auricular appendix 

 and pulmonary artery ; 9, placed between the innominate and left carotid 

 arteries ; 10, appendix of the left auricle ; n, n, the outside of the left 

 ventricle, the lower figure near the apex. (From Quain's Anatomy.) 



