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 trabecula j 

 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 gills) 



O.av. 



D.C.s 



Fct.x. Fa.d. 



FIG. 302. HEAKT OF Acanthis vnl- 

 yaris, FROM THE DORSAL SIDE, 



WITH THE ATRIUM CUT OPEN. 



(After Rose.) 



Co, tr, truncus anteriosus ; D.C.d 

 and D.C.s, right and left pre- 

 cavals ; O.av, atrio-ventricular 

 aperture ; V.a.d. and V.a.s, right 

 and left valve of the sinus ven- 

 osus ; la 4a, afferent branchial 

 arteries. 



