I.] THE FROG. 91 



a. The ventricle ; its single ventricular cavity and 

 thick spongy wall. 



b. The auriculo-ventricular aperture ; situated to 

 the extreme left (right of the preparation) of the 

 ventricle, and bounded by two auriculo-ven- 

 tricular valves. Raise the nearest of these, if 

 not already removed, and dissect it away. 



c. The atrium ; its thin wall ; its spacious cavity 

 completely subdivided into two chambers (r. and 

 1. auricles) by a longitudinal inter-auricular sep- 

 tum, lying well to the animal's left side. Ex- 

 amine the relations of this to the auriculo-ven- 

 tricular aperture; it is prolonged down on to 

 the valves which guard it, subdividing it into 

 two. 



a. The sinu-auricular aperture ; large and situ- 

 ated near the middle of the right auricle im- 

 mediately adjacent to the inter-auricular sep- 

 tum. Note its valvular margins. 



/?. The aperture of the pulmonary veins ; small 

 and rounded, opening near the top of the 

 left auricle, close to the septum. 



y. The auriculo-ventricular valve; note its cha- 

 racters and mode of attachment. 



//. The truncus arteriosus ; arising from the ex- 

 treme right-hand corner of the ventricle and 

 largely subdivided into two by a longitudinal 

 valve (cf. p. 1 8). Examine the aortic arches 

 in relation, passing bristles into them ; they 

 arise as under 



