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on its lateral and ventral surfaces. In the middle of its 
ventral wall is the sinu-auricular orifice through 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 (V. card.), the paired hepatic veins and the 
unpaired inferior jugular vein. 
The Auricle (Awr.) 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. 
‘wo of these valves are large, and are nearly anterior and 
posterior, whilst the third is much smaller, and is situated 
laterally. 
The Yentricle (Ven.) les ventral and posterior to the 
auricle. Its walls are very thick, and are produced 
internally into ridges—the column carnee, which 
largely reduce its cavity. It is separated by a deep con- 
striction from the bulbus arteriosus (.A4.), 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 semi-lunar 
valves. The wall of the bulbus is composed of fibrous 
connective tissue free from muscle fibres. It is very 
