THE HEART. 



215 



The frog's heart seen from the ventral surface, and 

 dissected so as to show its structure. The ven- 

 tral walls of the truncus arteriosus and of the 

 auricles and ventricle have been removed. From 

 a drawing by Mr. Hurst. 



appears to be absent (Fig. 135). Fig. 134. 



The septum is attached to the 

 walls of the auricles so as to 

 form two distinct cavities, which 

 communicate at the aviriculo- 

 ventricular opening-, where the 

 septmn has a free concave 

 border (Fig. 134). The right 

 am-icle is much larger than the 

 left, and receives the systemic 

 venous blood from the sinus 

 venosus by the opening (Figs. 

 133 and 134 SV.) already de- 

 scribed. The left auricle re- 

 ceives the blood from the lungs 

 by the pulmonary vein (Fig. 



133 PV.), which opens into the 

 auricle near the septum (Fig. 



134 PF.) : [the openings from 

 the sinus venosus and from 

 the pulmonary vein may be so 

 closely approximated as to be 

 separated by the septum only. 

 The septum is much thinner 

 than the auricular walls]. Both 

 auricles open into the ventricle 

 by the auriculo-ventricular open- 

 ing (Fig. 134). 



c. The ventricle (Figs. 133, 

 still living, is seen to be bluntly conical in 

 shape and darker on the left side than on the 

 right ; this is due to the ventricular wall of 

 the right side, near the truncus arteriosus, being- 

 somewhat thinner. The inner wall is not smooth 

 but possesses muscular ridges {Trahecnlae). At 

 its base the ventricle communicates with the 

 aui'icles by the auriculo-ventricular opening 

 (Figs. 134 and 136), and with the truncus 

 arteriosus by a separate opening (Fig. 134 A.). 

 The ventricle is lined by a layer of endocardial 

 endothelium. 



c. 



p. 

 p'. 



Auriculo-ventricular aperture and valve. 



Aperture leading from ventricle to truncus 

 arteriosus. 



Left carotid arch. 



Left auricle. 



Left pulmo-cutaneous arch. 



Style passed down right pulmo-cutaneous arch 

 into the truncus arteriosus, T. 

 P V. Opening of pulmonary vein. 

 RA. Right auricle. 

 S. Left systemic arch. 

 SA. Septum atriorum. 



SV. Opening of Sinus venosus into right auricle. 

 V. Ventricle. 



134, and 136 7.), examined while 

 Fig. 135- 



Dissection of a case in 

 which the auricular ■ 

 septum is placed more 

 to the left than is nor- 

 mal. 



S Septum. 

 SS Left auricle. 

 V Opening into the 

 Sinus venosus. 



