AMPHIBIA 765 free end of an oblique, but nearly horizontal septum, which divides the cavity of the synangium. From this thick parti tion thinner septa radiate to the walls of the synangium, which they thus divide into five longitudinal canals, of which that which lies to the right is twice as large as any of the others, In fact, it also becomes subdivided, further forwards, by a longitudinal septum, and then there are sk canals answering to the six aortic arches which spring from the synangium, where it reaches the anterior end of the pericardium. According to Hyrtl s account, the pylan- gium of Cryptobranchus has a very similar structure; but the synangium is completely split into two trunks, each of which contains three canals. This leads to the structure of the truncus arteriosus observed in the Frogs, which consists almost wholly of the pylangium. Three thick semilunar valves are placed at the ventricular end of this re- gion, and three others, also of un equal dimensions, at its synangial end. A longitudi nal ridge, with a rounded, free, ven tral edge, projects from the dorsal wall of the py langium. It is thicker anteriorly than posteriorly, and is directed obliquely, so that its anterior end passes into the right anterior valve, while its posterior extre mity is close to FIG. 23. The heart of .Sana pipi. tliA Ipft nrvefr>T-ir>r mi Valve. The ante- 1 c , i Dylan ffium (v^) a , . * are much larger than the posterior valves ; and, of the three anterior valves, that which lies on the dorsal side is the smallest. Imme diately beyond it is situated the aperture (p), which leads into the pulmonary trunks. In front of the pulmonary aperture is a wide cavity, whence the two great aortic trunks (Ao, Ao 1 ) spring. A tongue-like projection springs from the dorsal wall, and divides the cavity imperfectly. On the ventral side of the base of this tongue are the two open ings (Co) which lead into the carotid trunks. The three trunks carotid, aortic, and pulmonary pass out of the pericardium together, so closely united that they appear one. It is only at some distance beyond the pericardium that they separate, the anterior ending in the rete mirabile, which has received the name of the carotid gland;" the middle becoming the arch of the aorta; the posterior, the pulmo-cutaneous artery. 1 In the Peromela (e.g. JSpicrium), the heart presents many singular peculiarities (Fig. 24). In the first place, it is moved back to a distance which is relatively far greater than in any other Amphibia and in most lizards. Next, it is extremely elongated, and the truncus arteriosus is relatively more prolonged than any other part of the heart. But 1 The structure of the heart in the Amphibia has teen recently dis cussed with great ability, by M. Armand Sabatier in his Etudes sur le cceur Montpellier, 1873. The ventricle (K), the trnncus arteriosus, and the aortic trunks (.do.) are laid V cn from the ventral side - Spt- frce etl i? e of the septum auriculorum ; . semilunar valves at the ventricular end of the pylangium ; v 2 . valves at its synangial end; S, the septum of the pylangium; P< t ^ le a P erture of the pulmonary trunks; C, the apertures of the carotid trunks. the relative proportions of the pylangium and synangium are the reverse of those which obtain in the Anura, Tho two transverse rows of valves which mark the boundaries of the pylangium are situated close to one another, near the origin of the truncus, all the rest of which is made up of the synangium. A longitudinal partition, at first, divides the cavity of the synangium into two unequal passages; but, towards its anterior end, it contains four equal canals. Having reached the anterior extremity of the pericardium, the synangium divides, and the two pairs of canals become independent, but closely united, trunks, which run, on each side of the trachea, to about the level of the glottis. Here the two trunks join, and pass into the single arch of the aorta, which turns sharply back beneath the vertebral column. The carotid artery is given off from the junction of the two trunks with the single dorsal aortic arch. Shortly before the two trunks join, that upon the dorsal side gives off the pulmon ary artery. A single pulmonary vein opens into the left auricle; and it is worthy of notice, that the auricles and sinus are situated as far forwards on the dorsal aspect of the heart as in the Frogs. As regards the number and destination of the great vessels which arise from the synangium, great differences obtain in the different groups of the Amphibia. In the perennibranchiate Urodela, each of the three, or four, branchial arches has its appropriate aortic trank, which springs mediately, or immediately, from, the synan gium. The three anterior aortic trunks supply the gills, but are not wholly distri buted to them; so that the trunks which unite to form the dorsal aorta are derived partly from the gills and partly come di rectly from the ventral aorta. The anterior aortic arch gives off, on its ventral side, a hyomandibular artery to the walls of the oral cavity, which appears to represent the remains of the hyoidean and mandibular aortic arches, while, dorsally, it supplies the internal carotid. The pulmonary artery r , . f .-I f f-i J.--L FIG. 24. Ventral view of th is given on from the fourth aortic arch, or i, ea d and trunk of from the common trunk, which is formed by the union of this with those which pre cede it. In Cryptobranchus, according to Hyrtl, three trunks are given off on each side from the synangium. The most anterior cor responds with the hyomandibular artery of the perennibranchiate forms. The second belongs to the first branchial arch. It gives off no branch, but unites with the third and largest vessel to form a common trunk, which unites with its fellow beneath the vertebral column, and gives rise to the dorsal aorta. The posterior aortic arch gives off the pulmonary artery (which supplies a branch to the alimentary canal). From the common trunk a maxillary and an internal carotid artery are supplied ; while a third branch passes to the ventral side of the atlas, and, turning backwards, passes between the transverse process of the second and succeeding vertebrae as a collateral vertebral artery. In Salamandra, there are four aortic arches. The most anterior of these belongs to the first branchial arch. It gives off a hyoman dibular branch, then breaks up into a rete mirabile, whence the internal carotid artery is continued, and is connected by a mere ductus Botalli with the second arch. The second and third arches give off no branches ; but, along with the slender ductus Botalli of the fourth arch, coalesce into the trunk which unites with its fellow to form the dorsal aorta. Tha fourth arch gives off the crium glutinosum. Mn. mandible ; fly. hyoid ; Br Si- 2 , Sr 3 , branchial arches ; Gl. glottis ; Tr. trachea ; f.v.c. inferior vena cavn; V, ventricle; Au. auricles ; R.s.v.c., LJ.V.C. light and left superior cavte, T.a. trun cus arteriosus; Ao. left aortic arch; P. A. right pulmonary artery. The pericardium (lightly shaded) extends as far as the bifurcation of the
synangium.
