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on its lateral and ventral surfaces. In the middle of its 

 ventral wall is the sinu-auricnlar orifice throuo-h which its 

 cavity communicates with that of the auricle. This open- 

 ing is guarded by a rather weak valve consisting of two 

 membranous flaps, anterior and posterior in position, 

 which hang down slightly into the cavity of the auricle. 

 The large vessels opening into the sinus are the paired 

 precaval veins into which open the paired posterior car- 

 dinal veins (F. card.), the paired hepatic veins and the 

 unpaired inferior jugular vein. 



The Auricle {Aur.) lies dorsal and anterior to the 

 ventricle which it partly enfolds. Its external surface is 

 lobulated, the postero-dorsal portion being produced into 

 two notable lobes. Its walls are thin, but are strength- 

 ened internally, especially on their dorsal and ventral por- 

 tions, by interlacing muscle bands — the musculi pectinati. 

 A deep auriculo-ventricular groove separates it from the 

 ventricle. Its cavity communicates with that of the latter 

 by the auriculo-ventricular orifice, which is guarded by 

 three semi-lunar valves— pocket-shaped membranous flaps, 

 the cavities of which face the cavity of the ventricle. 

 Two of these valves are large, and are nearly anterior and 

 posterior, whilst the third is much smaller, and is situated 

 laterally. 



The Yentricle [Ven.) lies ventral and posterior to the 

 auricle. Its walls are very thick, and are produced 

 internally into ridges — the columnse carnese, which 

 largely reduce its cavity. It is separated by a deep con- 

 striction from the bulbus arteriosus {B.A.), which is a 

 flask-shaped dilatation of the proximal end of the ventral 

 aorta. Its cavity communicates with that of the bulbus 

 by an opening which is guarded by two strong f-emi-lunar 

 valves. The wall of the bulbus is composed of fibrous 

 connective tissue free from muscle fibres. It is very 



