188 THE VITAL FUNCTIONS. 



additional security against the retrograde motion of its fluid 

 contents. Valves are accordingly interposed between the 

 auricle and ventricle; and great refinement of mechanism is 

 displayed in their construction. Fig. 351 represents their 



appearance (at v) when the cavities, both of the auricle (D,) 

 and the ventricle (E) are laid open: c, c, as before, being the 

 upper and lower venaa cavse, and A, the main trunk of the 

 aorta. These valves are composed of two loose membranes, 

 the fixed edges of which are attached circularly to the aper- 

 ture of communication between the cavities, and their loose 

 edges project into the ventricle; so that they perform the 

 office of flood-gates, allowing a free passage to the blood 

 when it is impelled into the ventricle, and being pushed back 

 the moment the ventricle contracts; in which latter case they 

 concur in accurately closing the aperture, and preventing the 

 return of a single drop into the auricle. These valves being 

 attached to a wide circular aperture, it is necessary that they 

 should be restrained from inverting themselves into the 

 auricle, at each contraction of the ventricle. For this pur- 

 pose there are provided slender ligaments (which are seen 

 in Fig. 351,) fixed by one end to the edge of the valve, and 

 by the other to some part of the inner surface of the ventri- 

 cle, so that the valve is always kept within the cavity of the 

 latter. In the auricle, the same purpose is answered by the 

 oblique direction in which the veins enter it. 



