CIRCULATION IN THE VERTEBRATA. 261 



valves are composed of two loose membranes, 

 tlie 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 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 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 of the ventricle ; 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. 



The arteries themselves, especially the main 

 trunk of the aorta, as it issues from the heart, are 

 muscular, and when suddenly distended, contract 

 upon their contents. It was necessary, therefore, 

 to provide means for preventing any reflux of 

 blood into the ventricle during their contraction ; 

 and for this purpose another set of valves (r, Fig. 



