CH. XIX.] 



VALVES OF THE HEART 



213 



then pass by the septum around the other ventricle, and finally 

 terminate at the base of the heart : this is called the spiral layer. 



In the amphibian heart there is an obvious muscular con- 

 nection between the auricle and ventricle. A similar muscular 

 link mingled with non-medullated nerve -fibres also occurs in the 

 mammal. This bundle passes down from the interauriciilar 

 septum to the interventricular septum, where it divides into two 

 bundles, one for each ventricle : its fibres are peculiar in structure 

 and are known as Purkinje's fibres (p. 71). It is called the auriculo- 

 ventricular bundle. At the junction of the superior vena cava and 

 the right auricle is a focus of tissue known as the sino-auricular node 

 which corresponds to the sinus venosus of the embryo and lower 

 mammals. The special significance of these structures will be 

 discussed later (see p. 257). 



Valves. The arrangement of the heart's valves is such that the 

 blood can pass only in one 

 direction (fig. 194). 



The tricuspid valve (5, fig. 

 191) presents three principal 

 cusps or subdivisions, and the 

 mitral or bicuspid valve has 

 two such portions (6, fig. 192). 

 But in both valves there is 

 between each two principal 

 portions a smaller one : so 

 that more properly, the tri- 

 cuspid may be described as 

 consisting of six, and the 

 mitral of four, portions. Each 

 portion is of triangular 

 form. Its base is continu- 

 ous with the bases of the 

 neighbouring portions, so as 

 to form an annular mem- 

 brane around the auriculo- 

 ventricular opening, and is 

 fixed to the tendinous ring which encircles the orifice. 



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

 tendinous rings, their borders are fastened by slender tendinous 

 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 



FIG. 194. Diagrrm of the circulation through the 

 heart. (Dalton.). 



