. The biology of the frog . and appears paler than the rest. (2) The auricles lie immediately in front of the auricles are thin-walled and are separated from eachother internally by a septum, but from the outside theypresent only a faint indication of this division ; they areclearly separated from the ventricles by the coronary sulcus. (3) The bulbus cordis, lying in front of the right side of theventricle; it is a thickened muscular tube, extending ob-liquely across the right auricle; anteriorly it is continuedinto the thinner-walled truncus arteriosus from which it isdemarcated
. The biology of the frog . and appears paler than the rest. (2) The auricles lie immediately in front of the auricles are thin-walled and are separated from eachother internally by a septum, but from the outside theypresent only a faint indication of this division ; they areclearly separated from the ventricles by the coronary sulcus. (3) The bulbus cordis, lying in front of the right side of theventricle; it is a thickened muscular tube, extending ob-liquely across the right auricle; anteriorly it is continuedinto the thinner-walled truncus arteriosus from which it isdemarcated by a sulcus. (4) The truncus arteriosus issomewhat narrower than the bulbus; it soon divides intotwo diverging trunks, which give rise to three arteries, thecommon carotid, the aorta, and the pulmo-cutaneous. On the dorsal side of the heart is the triangular, thin- XV THE CIRCULATORY SYSTEM 265 walled sinus venosus; at the two anterior angles of the sinustheprecavalveins, or anterior vena cava, enter; the poste- eangl m ^. au,. FIG 72 -The heart of a frog cut open and seen from the ventral a bristle passed into the left carotid trunk; , aunculo-ven-tricular valves; b,b, bristle passed into the left systemic trunk; c,cbristle in left pulmo-cutaneous trunk; , carotid artery; ,carotid gland; cart, bulbus cordis (conus arteriosus of some authors) ;carJr, carotid trunk; Lau, left auricle; /^.a, lingual artery; /.*, longi-tudinal or spiral valve ; , pulmo-cutaneous trunk; , open-ing of pulmonarv veins; , right auricle; , smu-auncularaperture; sptaur, inter-auricular septum; z;, z/, valves; *, ventricle.(From Parker and Haswells Zoology.) rior apex receives the large postcavalvein, or posterior venacava. In front of the anterior margin of the sinus is thepulmonary vein, which empties into the left auricle. 266 THE BIOLOGY OF THE FROG CHAP. CCZ .CU The internal structure of the heart presents a complicatedand beautifully adapted mech
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