THE RIGHT ATRIUM 621 
caudalis) is at the posterior part, opposite to the fifth intercostal space. Between 
the two the wall pouches upward somewhat, and here the vena azygos opens. The 
coronary sinus opens ventral to the posterior vena cava; the orifice is provided with 
a small semilunar valve (Valvula sinus coronarii). The small coronary vein hasa 
separate opening in some cases close to that of the coronary sinus. The right 
atrio-ventricular orifice (Ostium atrio-ventriculare dextrum) is in the ventral part, 
— Anterior vena cava 
Right auricle 
Right 
F be atrium 
ovals 
(sinus 
venarum) 
Posterior 
vena cava 
Orifice of P 
coronary onus 
sinus arleriosus 
Chorde 
tendinee 
Right ventricle 
(septal wall) 
Fic. 545.—Ricut Sie or Heart or Horse Openep tp py Removat oF GREATER Part or RicHT WALL. ORGAN 
HARDENED in situ. 
The right ventricle was in diastole. r, Intervenous crest; V, V, tricuspid valve; P, P, papillary muscles; m, m, 
moderator bands. Arrow points into originof pulmonary artery. The crista supraventricularis, which separates the 
atrio-ventricular orifice from the conus arteriosus, is shown but not marked. The crista terminalis (unmarked) is 
nearly directly dorsal to the preceding. 
and leads into the right ventricle. In addition to the foregoing there are several 
small orifices of the ven cordis parve; these are concealed in the depressions 
between the musculi pectinati. 
The atrium is lined with a glistening membrane, the endocardium. Its walls 
are smooth except on the right and in the auricle, where it is crossed in various 
directions by muscular ridges, the musculi pectinati. Small bands extend across 
some of the spaces enclosed by the musculi pectinati. The latter terminate above 
