88 THE FROG chap. 



distinct auricles, the right {r. au), considerably larger than the 

 left (/. au), separated from one another by a vertical partition 

 (spt. aur). 



You have already seen that the conus arises from the 

 right side — the frog's right, not yours — of the base of the 

 ventricle. A little to the left of this point there is an 

 aperture through which a bristle can be passed from the 

 ventricle into either of the auricles. Both auricles, then, 

 communicate with the ventricle by a single auriculo- 

 ventricular aperture. This is guarded by two little mem- 

 branous flaps (au. V. v), which spring, one from the dorsal, 

 one from the ventral edge of the aperture, and hang down 

 into the ventricle, to the walls of which they are attached 

 by little tendinous cords, represented in the figure by white 

 streaks. Thus the flaps have the character of folding doors 

 or valves opening only one way ; they readily flap backwards, 

 i.e., into the ventricle, but are prevented from flapping 

 forwards or into the auricles by the tendinous cords 

 attached to their backs. The two flaps are the auricula- 

 ventricular valves. Their mode of action is easily understood. 

 If the auricles, being full of blood, contract and squeeze 

 themselves together, the pressure will force aside the valves 

 and allow the blood a free passage into the ventricle. On 

 the other hand, if the ventricle contracts, the blood, getting 

 behind the valves, will force them together and close the 

 aperture, the tendinous cords preventing their being driven 

 into the auricles by undue pressure. 



In the interior of the conus is a longitudinal fold or valve 

 {l.v), which traverses it obliquely, attached to its dorsal wall 

 and free ventrally ; and there are also three little semilunar 

 or watch-pocket shaped valves {v) guarding the aperture 

 between the ventricle and the conus : they are arranged with 

 their edges turned forwards or towards the cavity of the 



