Where the posterior extremities are less or not larger than the anterior ones, as in the Ursine Dasyure and Wombat, the posterior cava is somewhat less than the left vena innominata (figs. 131 and 132), and they appear to terminate by separate apertures in the auricle; but in the Kangaroo (fig. 131) the proportions of the two veins are reversed, and the posterior cava more obviously receives the left vena innominata before it terminates: these two veins meet at a very acute angle, and are separated by a crescentic ridge similar to, but thinner than, that which divides their common orifice from the orifice of the anterior cava.
The right auriculo-ventricular valve is mem branous, as in the placental Mammalia, and its free margin is attached by fine chords ten diner to three columns crimes; these in the Kangaroo (fig. 131) all arise from the septum of the ventricles, but in the Wombat (fig. 132) The ventricles and auricles present the usual mammalian proportions and relative thickness of their parietes. Three sigmoid valves arc situated at the origin of the pulmonary artery, and the same number at that of the aorta.
After the coronary arteries, the primary branches from the arch of the aorta rise in some species by three, in others by two trunks. The broad-chested Marsupials, the Koala and Wombat for instance, are those in which the left carotid (g', fig. 132) and subclavian (h') arise separately from the arch ; the arteria inno minata dividing into the right subclavian and carotid (g, h, fig. 132), as in man. In most of the other Marsupials the innominata gives off both carotids (g, g, jig. 131) as well as the right subclavian ( h ); and the left subclavian alone has a separate origin. The common carotid in the Kangaroo gives off the thyroid artery, and afterwards divides opposite the transverse process of the atlas into the external and internal carotids. The internal carotid describes a sharp curve at its origin, and passes along the groove between the occipital condyle and the exoccipital process to the foramen Ca roticum. The vertebral arteries are given off by the subclavians, and pass to the skull, as usual, through the vertebral foramina of the cervical transverse processes. They unite be neath the medulla oblongata to form the basilar artery, which sends off at right angles to the cerebellum two branches as large as itself: it divides opposite the anterior margin of the pons Varolii, and the diverging branches are connected by two straight transverse canals, before they anastomose with the internal caro tids to form the circle of Willis. No pecu liarly marsupial condition occurs in the distri bution of the other arteries of the head, or those of the neck, the chest, and anterior ex tremities; but I may observe that in the Koala, Wombat, Kangaroos, Potoroos, most I'halan gers, ( Phal. Cookii is an exception,) most Pe
taurists, ( Pet. Sciurcus is an exception,) the Opossums, Bandicoots, and Phascogales, the brachial artery perforates the internal condyle of the humerus; it passes over that condyle, impressing it with a more or less deep groove in the Dasyures and Thylacine.
In the abdomen, the primary branches of the aorta are sent off in the same order as in most of the ordinary Mammalia, with the ex ception of the constant absence of an inferior mesenteric artery. This modification probably relates to the simplicity of the mesenteric at tachment of the intestines above described. A still more marked example of the oviparous affinities of the Marsupialia, as exemplified in the arterial system, occurs in the mode of origin of the great arteries of the posterior extremities. In Man and the ordinary Mam malia these are derived, as is well known, from a single trunk on each side—the common iliac artery; in Birds from two primary branches of the aorta, one corresponding with the external iliac and femoral, the other with the internal iliac and ischiadic arteries. In the Kangaroo and Phalangista vulpina the aorta gives off, opposite the interspace of the two last lumbar vertebra, the iliac arteries ; but these are after wards resolved into the ordinary branches of the external iliac of the placental Mammalia, with the addition of the ilio-lurnhar artery. The trunk of the aorta, much diminished in size, maintains its usual course for a very short distance, and then gives off the two internal lilacs, and is continued as the arteria sacra media' to the tail. The transitional cha racter of this part of the marsupial sangui ferous system between the oviparous and pla cental types, is manifested in the large size of the external iliacs as compared with the internal iliacs, their greater share in the su ply of blood to the hinder extremities, and the brevity of the aortic trunk between their origins. In most Birds the femorals or external iliacs are smaller than the ischiadic or internal iliac arteries subsequently given off. At the upper part of the thigh the femoral artery divides into two equal branches; the one which corresponds with the radial artery in the fore leg (nr, fig.134 ) principally supplies the foot in the Kangaroos ; It passes along the back of the radius, between the gastrocnemius internus and tibialis posticus, and divides a little above the internal malleolus. The smaller division (1, fig. 134), which follows the ord i nary con rse of the femoral along the popl teal space, is lost upon the inner and posterior part of the tarsus; the larger branch winds over the malleolus to the front of the tarsus, sends off the anterior tarsal artery, and is then continued along the inner and afterwards the under part of the metatarsal bone of the long and strong toe.