AMPHIBIA. 



195 



the usual type, and larger and more numerous red corpuscles, 

 which owe their colour to haemoglobin. They are oval and flat, 

 with a large central nucleus of similar shape, projecting some- 

 what on both surfaces of the corpuscle. The blood circulates in 

 a closed system of tubes, consisting of heart, arteries, veins, and 

 capillaries. 



The heart (Figs. 51 and 53) is situated in the anterior part 

 of the thoracic region, on its ventral side. It is enclosed in a 

 membranous bag, the pericardium, which is composed of an inner 

 layer closely adherent to the heart, and a loose outer layer. A 

 space, the pericardia! cavity, which is a separated part of the 

 co.ilom, is found between the two. The heart is roughly conical 



c.s.d. 



---A.d. 



ci 



Fig. 53. HEART OF FROG (after Ecker), slightly enlarged. a, Ventral 

 view ; b, dorsal view, siuus venosus opened ; s.v, sinus venosus ; Ap, 

 its opening into Ad, right auricle ; As, left auricle ; V, ventricle ; 

 B, truncus arteriosus ; 1, 2, 3, carotid, systemic, and pulmo- 

 cutaneous arches; c, carotid gland; c.s.d and c.s.s, right and left 

 precavals ; c.i, postcaval ; v.p, pulmonary vein. 



in shape, and its backwardly-directed apex fits into a notch in 

 the liver. A dark coloured triangular sac, the sinus venosus, with 

 thin walls, lies on the dorsal side of the heart, and its base is 

 formed by two thin-walled, dark-red auricles, right and left, while 

 a muscular, paler ventricle makes up its apical region. A 

 muscular tube, the truncus arteriosus, leading from the right side 

 of the ventricle, is closely applied to the ventral surface of the 

 right auricle. 



The cavity of the right auricle is separated by a thin muscular 

 partition, the auricular septum, from that of the left auricle. Into 

 the dorsal Avail of the former cavity, near the septum, the sinus 

 venosus opens by an oval valvular aperture. There is a some- 



