Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/807

Rh AMPHIBIA 763 median aperture ; tins is relatively wider in Dactylethra. Two grooves in the mucous membrane of the roof of the mouth pass from the Eustachian to the posterior nasal apertures, and enclose a lyrate space, in these genera. The tongue is rudimentary in .the lower Urodela; but, in the Salamanders, it may be free, fleshy, and even mushroom-shaped. In Pipa, and Dactylethra, no trace of a tongue is to be observed. In liana, as in most Anura, the anterior end of the tongue is com paratively small and little elevated above the mucous membrane of the floor of the mouth, but the posterior end is produced into a free fleshy mass, bifurcated at its extremity. It is this free end which is thrown forward- in the act of prehension, the tongue turn ing on its anterior end as on a hinge. Rhinophrynus is the only Anuran in which the anterior end of the tongue alone is free. In the males of many Anura the mucous membrane of the mouth is produced outwards, on each side, between the mandible and the liyoid, into a sac, which becomes iilled with air, and gives rise to a conspicuous projection of the integument of the throat. In some cases these two sacs coalesce into one. Salivary glands have not been discovered in any Amphibia. Except in the Peromela, the gullet is short. It passes into an elongated stomach, the long axis of which coincides with that of the body in the Urodela and Peromela, but becomes oblique, or transverse, in the Anura. The intestine is never very long, and, consequently, its convolutions are few and simple. There is always a marked distinction between the small and the large intestine. The latter opens into a cloaca, which receives the ducts of the urinary and genital apparatus. The stomach and intestine are enclosed in peritoneum, and suspended to the roof of the abdominal cavity by a mesenteric fold. The liver is always provided with a gall-bladder. It is distinctly bilobed in most Anura; and, in Pipa and Dacty lethra, the two lobes are completely separate, the gall-bladder being attached to the right lobe. In the Peromela, the liver has an ex ceptional form, being divided into a great number of small lobes, arranged in a longitudinal series so as to overlap one another. A pancreas is always present ; but sometimes, as in liana, it is small, and its glandular substance surrounds the hepatic duct. The spleen, enclosed in the mesentery, is elongated in the Urodela and Peromela, rounded in the Anura. The Organs of Circulation. The heart is contained within & pericardium, the walls of which generally exhibit numerous scat tered pigment z &amp;lt;.; cells, and though delicate in the fresh state, are pt to become tough and almost pergamentaceous in spirit speci mens. The heart (if we apply that name to the whole apparatus enclosed within the pericardium, except the venae cavae), presents a series of five segments, to which, enumera ting them from behind forwards, the following terms may be applied : 1 , The sinus venosus ; 2, the atrium; 3, the ventriculus; 4, the pylangium (from TnAwv, a gateway, and dyyeZov, a vessel) ; and 5, the synangium. Atrium here denotes the auricular division of the heart, comprising the right and left auricles. Pylangium and synangium, together, are the equivalents of that portion of the heart which lies between the ventricle and the anterior wall of the pericardium, and which has been variously named bulbus, cavus, and truncus, arteriosus. These five segments of the heart are so arranged, that the sinus and atrium lie on the dorsal and posterior aspect of the organ, while the others occupy its ventral a*nd anterior region. Viewed sideways, in fact, the heart has the shape Fio.15. The heart of Stredon mextcanus. Lateral view of the heart contained within the pericardium, the left wail of the sinus venosus and of the auricles bcin? removed ; &amp;lt;S. sinus venosus ; /. v. c. inferior vena cava ; L. s. v. c. left superior vena cava; L.A. left auricle; R. A. right auricle ; Spt. septum auriculorum ; V. ventricle; T. a. truncus arteriosus; 1, 2, 3, 4, the aortic arches. The arrow traverses the ?inu-auricular aperture. The auricuJo-ventiicular aperture lies to the right of the arch formed by the free edge of the septum. JiS.v.c. LV.&. Td. X.S.v.e. of a ^, ot which the sinus and atrium occupy the tipper, and the other segments the lower half. But it also always presents, more or less, a 7 lateral flexure, between its an terior and poste rior points of adherence to the middle line of the pericar dium; so that, viewed from above, it ap proximates the form of an N, of which the right half is repre sented by the synangium, py langium, and FIG. 16. Ventral view of the same heart contained in the V n-n trip IA a n c pericardium. R.s.v.c, right superior vena cava. The veiJ ,16, and pylangium laid open to show the two transverse rows the left half by f valves, V 1, V 2. The commencement of the synan- . . i gium (So.), cut across. the atrium and sinus. The pylangium, in fact, always arises from the right side of the ventricle, while a large part of the atrium arid of the sinus very often lies to the left of the ventricle, the auriculo- ventricu lar aperture of the ven tricle looking to the left side and forwards. There is an interest ing difference to be observed in the relative position of these seg ments of the heart in the lower and the higher Amphibia. In Siredon, for example, the greater part of the sinus lies completely behind the ventricle, F]G 17 _ Posterior view of the Mine hearti re. and the Sinu-auncular moved from the pericardium ; P.v. pulmonary aperture is situated on vem- the posterior face of the atrium, on a level with the posterior part of the ventricle ; but, in the Frogs, the sinus lies altogether above the ventricle, without sensibly projecting behind it, and the sinu- auricular opening lies in the dorsal face of the atrium, in front of the level of the auriculo- ventricular aperture. In other words_, the seg ments of the heart have a less marked vertical flexure in the lower, than in the higher Am phibia, and more nearly approach the condition of the embryonic heart. nme heart laid &S.V.G,, JPA-- SJit. Tn corresoondence with FlG - 18 - The atrium of the sn . open and its walls spread out, so as to show this, the Superior Cavse the opening of the pulmonary vein, P.v., and traverse the pericardium thc &quot;^-auricular P erture - * to enter the sinus near its posterior end in Siredon, but about its middle in the Frog. The sinus venosus is a thin walled sac, which is relatively largest in the lower Amphibia and smallest in the Frogs. Anteriorly, it usually receives, on each side, one of the two superior venae cavse ; posteriorly, the single vena cava