part of part of in left side of embryonic structure sinus venosus left horn of sinus venosus FMA:70321 EMAPA:16238 sinus venosus left horn VHOG:0000488 EHDAA:482 UBERON:0005091 Three major adaptations, or 'novel cardiac components', that were not present in the ancestor chordate heart tube can be distinguished in the lower vertebrate heart: the atrium, ventricle, and possibly the muscular sinus venosus.[uncertain][VHOG] EHDAA2:0001840