764 AMPHIBIA Sjit. inferior opens into it. But, in some cases (as in Meno branchus and Pipa, according to Meyer) the inferior vena cava divides in to two branches, each of which coalesces with the superior vena cava of its sidebef ore open ing into the sinus. The supe rior cavos may open into the Vc sinus immedi- Flr " 19 -~ The Heart of Rana esculenla. Lateral view of the i ti. AT, heart contained within the pericardium. (The heart has ately alter they been carefully drawn to scale in situ, and the parts shown V,r, V p t r ,i v nrqnrl by dissection put in as if the organ were transparent.) nave tiaveisuu s _ ginus venusus . /<. inferior vena cava; S.i:c.l. left the pericardi- superior cava; /SU-.c.r. opening of the right superior it- cava; P.V. pulmonary vein (its dotted contour is seen Uni, as in MenO- through the left superior cava); c, style introduced into branchus Or * ie P u monar y vein and passing into the left auricle; 6, style introduced into the sinu-auricular aperture and they may be passing into the right auricle (R.A.), where its end is olin-rf- t-rn-nVa r>q visible to the right of the septum, Spt. Yc. ventricular LILK.B, ds ca vity. T.a. truncus arteriosns. Ao. aortic arch; a, in the FrO^S or ligament passing from the wall of the pericardium to the . , ventricle. as in Siredon and the Salamanders, the right cava may be long and the left short. The sinu-auricular aperture, by which the sinus and the right auricle communicate, is small, relatively to the size of these two cavities, and Las an oval form. Its lips may be slightly prolonged towards the cavity of the auricle, but do not give rise to very de finite sinu - auricular valves. The auricular seg ment of the heart, or atrium, is always more or less bi-lobed, the truncus arteriosus being embraced by ., , , J f FIQ. 20. Ventral view of the heart of Rana escu- the two lobes, One Ol Itnta, cbtained in the same way. A style is vliir-li nrm pptsi rm ita passed through the aperture which leads from ,n projects on it the V(ntricls into the p y i anBium . L , A _ i uft tight Side and the auricle; Ao. Ao*. aortic arches; R.s.v. c. right, Other On the left. The ^di.^.c. left superior cava. right and left lobes are equal in Proteus; both lobes are large and sacculated in Siren; in Menobranchus, the left lobe seems to be large, in Epicrium, the right; but many of these differences are pro bably accidental. In the Bullfrog (Rana pipiens} the two lobes of the atrial segment of the heart com pletely envelop the truncus arteriosus, and become united together by fibrous tissue, which connects their walls on the ventral side of the truncus. The atrium is usually divided into two cavities, of which the left is smaller than the right, by a Septum, which CX- FlG - 21 - The heart of Ran/jeseu^nta,vaw- 4- A t 4-1, i fi. 11 t e(1 from above and behind. The aortic tends irom the lelt Wall Of arches seen through the auricles. L.p.v., the atrium towards the R-^-"- left and right pulmonary veins. auriculo-ventricular aperture. The cavity of the auricular segment thus becomes divided into a smaller, left, auricle, which, lies behind and to the left of the septum, and a larger, right, auricle, to the right and in front of the v. Z.fl.V. M.Ji.-v. J-.V.C.- still com- FlG - 22 - Thc lc ft auricle of the bullfrog (Runaptpien?) laid open in such a manner as to show the septum with its nerves (ii) and ganglia (g), and the manner in which it descends upon the free surfaces of the auriculo-ventricular valves, F 1 , V 2 . P.v. opening of the pulmonary vein; L.s.v.c. left superior vena cava; F. ventricular cavity. septum. In the Frogs, the septum auriculorum is a com plete partition, containing muscular fibres, and the septal branches, with their ganglia, of the cardiac nerves of tho pneumogastric. j> u It divides the auriculo-ventricu lar aperture, pass ing from one au- riculo - ventricular valve to the other, and ending be tween them by a free edge, which might almost be said to lie in the cavity of the ven tricle (Fig. 23). In Lissotriton punctatus, and in Siredon, the sep tum, plete, ends in the cavity of the au ricular segment by a free edge, which arches over the auriculo-ventricular aperture. In Meno branchus, the septum is reduced to little more than a wide-meshed network of branched muscular bands, and, in Proteus, the existence of a septum is doubtful. The auriculo-ventricular aperture is always situated at the left side of the posterior end of the auricular segment, where the latter joins the ventricle. In Rana esculenta and pipiens it possesses distinct, though short, membrane ous valves, the free edges of which, directed towards tho ventricular cavity, are kept down by fine tendinous filaments. The common trunk, formed by the union of the two pulmonary veins, runs over the dorsal wall of the sinus venosus, passes between the two superior cavas, and, usually dilating, opens into the cavity of the left auricle, close to the sinu-auricular aperture; and, in fact, separated from it only by the septum, which continues the direction of the right wall of the pulmonary vein. The ventricular segment always has thick walls and a comparatively small cavity, which lies in the anterior half or base of the ventricle, and takes a direction from left to right, or from the auriculo-ventricular aperture to that of the truncus arteriosus. In consequence of the loose and spongy texture of the greater part of the thickness of the ventricular wall, it must be recollected that its apparent cavity by no means represents its capacity. The truncus arteriosus of Menobranchus is subcylindrical, in that half which is nearest the ventricle, but, in the other half, has a dilated and ovoid form. The latter, in reality, consists of the origins of the aortic arches, closely united together (synangium), while the former subdivision is the gateway between the ventricle and the great vessels, or the pylangium. It presents two transverse rows of semilunar valves, three in each row; the lower or posterior row being close to the opening of communication between the pylan gium and the ventricle, while the other row is near the anterior end of the pylangium. IiSiredon(Figs. 15 and 1G) there is the same division into a pylangium proper and a large oval bulb-like synangium, formed by the united aortic arches. Three valves in a transverse row are situated at each end of the pylangium. An oblique ridge projects from the dorsal wall of the pylangium, beginning low on the left side, and gradually increasing in size, until it passes into the dorsal valve of the anterior row. There is a small space in front of the
anterior row of valves, into which projects the posterior
