478 BANGUI-FERGUS SYSTEM. 



of plexuses of parallel blood vessels, derived from the portal 

 vein, the hepatic artery, and the hepatic vein, the minute 

 lobules of the organ being formed by straight blood canals 

 opening into sinuses, like the numerous approximated, pa- 

 rallel, pancreatic follicles of osseous fishes. The hepatic vein 

 sometimes forms a distinct sinus between the diaphragm and 

 the liver, as in the rays, where there are also two lateral pos- 

 terior venae cavae with enlargements in their course. The 

 two anterior venae cavse extend, like jugular veins, along the 

 sides of the neck, receiving the venous blood from the head,, 

 the pectoral fins, and the anterior parts of the trunk, and 

 frequently form a sinus, by their union behind the head, be- 

 fore they open, with the posterior cavae and the hepatic veins, 

 often three in number, into the common sinus venosus, be- 

 tween the pericardium and the tendinous diaphragm. In 

 many fishes there are two hepatic veins, and in others they 

 unite into a single trunk, as in most higher animals. So 

 that the vascular system of fishes is advanced not only by 

 the development of a distinct chyliferous and lymphatic sys- 

 tem of vessels, but also by that of a more or less extensive 

 hepatic and a renal circulation of venous blood, which have 

 not been traced in the invertebrated classes. 



The amphibious animals commence their career as fishes, 

 having in their tadpole state but one auricle and one ven- 

 tricle of the heart, breathing by means of gills, and send- 

 ing the whole of their blood through the branchial arches 

 of the aorta as in the adult state of the finny tribes, and as in 

 the embryos of all higher vertebrata. The heart of the tad- 

 pole, like that of the fish, consists of a thin capacious but 

 muscular auricle, which receives the entire venous blood of 

 the system, and a smaller muscular and commonly rounded 

 ventricle, which propels it into a distinct contractile fleshy 

 bulbus arteriosus, as seen in the larva of the triton cristatus, 

 (Fig. 136. a.) From the bulbus arteriosus, or commence- 

 ment of the great aortic trunk, the venous blood is conveyed 

 by three or more lateral branches, or branchial arches, on 

 each side, (\36.b,b,b,) both into the first and more deci- 

 duous external branchiae, and into the later and more durable 

 internal forms of these organs, as into the deciduous external 

 filamentous and the permanent internal laminated branchiae 

 of plagiostome fishes. The number of branchial arteries on 



