CIRCULATION IN THE HIGHER CLASSES. 239 



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 venae 

 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 

 aperture 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 pre- 

 venting the return of a single drop into the auricle. 

 These valves being attached to a wide circular aper- 

 ture, it is necessary that they should be restrained 

 from inverting themselves into the auricle, at each 

 contraction of the ventricle. For this purpose 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 



