350 DISSECTION OF THE RABBIT. 



1. The auricles. 



Cut away the outer walls' of both auricles so as to expose 

 cavities. Wash out the contained blood, 



a. The right auricle. The wall of the auricle is thin ; 

 that of the auricular appendix is thicker, and 

 is marked internally by muscular ridges, 

 i. The orifices of the vense cavse. 



The right anterior vena cava opens into 

 the anterior part of the auricle. 



The left anterior vena cava opens into the 

 left side of the posterior end of the auricle : 

 just before its opening it receives the coronary 

 vein, returning blood from the substance of 

 the heart. 



The posterior vena cava opens into the 

 dorsal surface of the auricle, in front of the 

 left anterior vena cava. 



ii. The Eustachian valve is a membranous fold, 

 between the orifices of the left anterior vena 

 cava and of the posterior vena cava. During 

 foetal life it directs the blood from the pos- 

 terior vena cava through the foramen ovale 

 into the left auricle. 



iii. The septum auricularum is a thin partition 

 between the right and left auricles. In it is 

 an oval patch, the fossa ovalis, close to the 

 opening of the posterior vena cava. This is 

 thinner than the rest of the septum, and 

 is perforated in the embryo by the foramen 

 ovale, which sometimes persists in the adult, 

 and is guarded by a membranous valve with 

 chordae tendinesB. Even when the foramen is 

 closed, the notched edges of the valve, and 

 remnants of the chords tendinefe are often 

 recognisable. 



iv. The right auriculo-ventricular aperture is a 



