32 THE VASCULAR SYSTEM OF THE FROG. 



2. The Auricles. Turn the heart over, with its ventral surface 

 uptvards. Cut away the ventral ivall of both auricles with fine 

 scissors, talcing care not to damar/e the truncus arteriosus ivhich 

 lies across the right auricle. Wash out the blood from the auricles. 



"b. The right auricle (Fig. 6 RA) is the larger of the two. 

 It has thin walls, thickened by muscular strands 

 which form interlacing reticular ridges on its inner 

 surface. In the dorsal wall of the auricle, very near 

 the median plane of the heart, is the aperture from 

 the sinus venosus already described (Fig. 6 SY). 



h. The left auricle (Fig. 6 LA) is smaller, sometimes 

 much smaller, than the right auricle, which it resem- 

 bles in the structure of its walls. In its dorsal wall, 

 very close to the sinu-auricular aperture, is the opening 

 of the pulmonary vein (Fig. 6 PV). 



c. The inter auricular septum is the thin partition between 

 the right and left auricles. It is much thinner than 

 the walls of the auricles, and is placed somewhat 

 obliquely, the left auricle lying rather more dorsally 

 than the right. The septum ends with a free posterior 

 edge, opposite the auriculo-ventricular aperture. 

 Cut aivay ivith scissors the ventral wall of the ventricle, taking 

 care not to damage the truncus arteriosiLs. 



3. The Ventricle (Fig. 6 V) is conical in shape with the 

 apex backwards, and has a small central cavity, with thick 

 spongy walls. The spongy character is due to great develop- 

 ment of a reticulum of interlacing muscular strands similar to 

 those of the auricles : the true outer wall of the ventricle is no 

 thicker than that of the auricles, and the meshes of the sponge- 

 work are really part of the cavity of the ventricle, and are filled 

 with blood. 



The auriculo-ventricular aperture lies at the base of the 

 ventricle, and rather to the left side. It is guarded by valves 

 (Fig. 6, A) Avhich hang into the ventricle, and are tied down at 

 their edges by fine tendinous threads ; and it is divided by the 

 free lower edge of the interauricular septum into right and 

 left divisions, admitting blood from the right and left auricles 

 respectively. 



C^it away carefully, tvith fine scissors, the ventral wall of the 

 truncus arteriosus so as to expose its cavity and the contained valves.. 



