CIRCULATION OF THE BLOOD. 109 



veins and taken to the left auricle. From the left auricle it passes into 

 the left ventricle, and thence into the aorta, by which it is distributed to 

 the capillaries of every portion of the body. The branches of the aorta, 

 from being distributed to the general system, are called systemic arteries; 

 and from these the blood passes into the systemic capillaries, where it 

 again becomes dark and impure, and thence into the branches of the 

 fii/xfanii' veins, which, forming by their union two large trunks, called 

 the superior and inferior vena cava, discharge their contents into the right 

 auricle, whence we supposed the blood to start. 



The Valves of the Heart. The valve between the right auricle 

 and ventricle is named tricuspid (5, Fig. 99), because it presents three 

 principal cusps or subdivisions, and that between the left auricle and ven- 



FIG. 97. Diagram of the circulation through the heart. (Dalton.) 



tricle bicuspid (or mitral), because it has two such portions (6, Fig. 93). 

 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, portions. 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 neighboring portions, so 

 as to form an annular membrane around the auriculo -ventricular open- 

 ing, being 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 middle-piece, 

 extending from its base to its apex, and including about half its width, 

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

 or edges. 



While the bases of the several portions of the valves are fixed to the 



