. The anatomy of the frog. Frogs -- Anatomy; Amphibians -- Anatomy. 214 THE VASCULAR SYSTEM. and so forms a closed lymph-sac. The pericardium is more or less pigmented^ and contains both vessels and nerves ; it may contain fat-cells]. B. The Heart. [The heart is a hollow muscular organ, composed of : ( i ) two auricles, forming- the wider anterior portion ; (2),the ventricle, placed behind the auricles ; (3) the sinus venosus, situated clorsally ; and (4) the truncus arteriosus, lying ventrally. Examined while still living, the auricles are seen to be much darker in colour than the ventricle,
. The anatomy of the frog. Frogs -- Anatomy; Amphibians -- Anatomy. 214 THE VASCULAR SYSTEM. and so forms a closed lymph-sac. The pericardium is more or less pigmented^ and contains both vessels and nerves ; it may contain fat-cells]. B. The Heart. [The heart is a hollow muscular organ, composed of : ( i ) two auricles, forming- the wider anterior portion ; (2),the ventricle, placed behind the auricles ; (3) the sinus venosus, situated clorsally ; and (4) the truncus arteriosus, lying ventrally. Examined while still living, the auricles are seen to be much darker in colour than the ventricle, due to the blood being seen through their thinner walls ; between the auricles and the ventricle is a distinct groove, the auriculo- ventricular groove. The whole organ is more or less pigmented, especially the ventricle {E. temjjoraria has much less pigment).] a. The Sinus venosus [Simis venarum cavarum), (Fig. 133 SV.) is a thin-walled sac lying on the dorsal surface of the heart, and very slightly to the right side (Hoffmann). It is somewhat triangular in form, receiving an anterior caval vein at each anterior angle {V.), and the posterior caval vein (/C.) at the poste- rior angle. The ventral wall has a transverse opening {SF'.), by which it communicates with the right auricle ; the opening Tlie heart, seen from above, and dissected to show the pOSSCSSCS twO ValvCS, an ante- rior and a posterior, and is placed close to the auricular septmn. b. The auricles [are not always separated, as the sep- tum which usually brings about the separation is in some cases incomplete, and in rare in- stances appears to be absent (Hoffmann)] ; according to Eeker, the auricular septum is sometimes placed so far to the left side that it r-SV. PV. opening from the Sinus venosus to the right auricle ; after Howes. C". IC. LA. P'. PV. RA. S'. SV. s v. Right carotid arch. Posterior caval vein. Left auricle. Right pulmo-outaneous arch. Pulmonary vein. Right auricle. Right systemic arch. Sinus
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