. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. 400 COMPARATIVE ANATOMY in. THE HEART, TOGETHER WITH THE ORIGINS OF THE MAIN VESSELS. Fishes (including Cyclostomes).—The heart in Fishes is situated in the anterior part of the body-cavity, close behind the head. It consists of a ventricle, with a truncus arteriosus or merely a bulbus (Cyclostomi, most Teleostei), and an atrium or auricle, the latter receiving its blood from a sinus venosus, and being laterally expanded to form the appendices auriculae (Figs. 302 and 303.) In correspondence with the work which each porti
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. 400 COMPARATIVE ANATOMY in. THE HEART, TOGETHER WITH THE ORIGINS OF THE MAIN VESSELS. Fishes (including Cyclostomes).—The heart in Fishes is situated in the anterior part of the body-cavity, close behind the head. It consists of a ventricle, with a truncus arteriosus or merely a bulbus (Cyclostomi, most Teleostei), and an atrium or auricle, the latter receiving its blood from a sinus venosus, and being laterally expanded to form the appendices auriculae (Figs. 302 and 303.) In correspondence with the work which each portion has to perform, the walls of the atrium are comparatively thin, while those of the ventricle are much stronger, its muscles giving rise in the interior to a muscular network in which a series of larger trabeculaj can usually be recognised: this holds good throughout the Crani- ata. Between the sinus venosus and atrium, and also between atrium and ventricle, membranous valves are present; there are primarily two atrioventricular valves, but they may become further subdivided. Numerous valves, arranged in rows, are present in the muscular conus arteriosus (Fig. 303, A): these are most numerous in Elasmo- branchs and Ganoids. There is a tendency, however, for the posterior valves, or those which lie nearest the ventricle, gradually to undergo reduction (B). Only the most anterior row persists between the ventricle and bulbus in Cyclo- stomes and most Teleosts (c), but amongst the latter two rows are retained in the vestigial conus in Albula (Butirinus) and Tarpon. The heart of all Fishes ex- cept Dipnoans contains venous blood only, which it forces through the afferent branchial arteries (Figs. 302, 320, 321) into the capillaries of the gills, where it becomes oxygenated, to pass thence into the efferent branchial arteries, and so into the dorsal aorta. The heart of the Dipnoi (Figs. 304 and 305), in correspondence with the double mode of respiration (by lungs as well as by g
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