THE EIGHT 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 has a 

 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, 



Orifice of vena azygos 



Fossa 

 ovalis 



Posterior 

 vena cava 



Orifice of 

 coronary ^- -^^ 

 sinus % 



Chorda; 

 tendinece 



interior vena cava 



Right auricle 



Right 

 atrium 

 (sinus 

 venarum) 



Conus 

 ■j arteriosus 



Right ventricle 

 (septal imdl) 



Fig. 545 



-Right Side of Heart of Horse Opened up by Removal of Greater Part of Right Wall. 



Hardened in situ. 



Organ 



The right ventricle was in diastole, r, Intervenous crest: V, V, tricuspid valve: P, P, papillary muscles: m, m, 

 moderator bands. Arrow points into origin of 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 vense cordis parvse; 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 



