CIRCULATION IN FISHES. 



245 



the central organs are seen to consist of four 

 cavities (c, d, e, f), opening successively the one into 

 356^ the other. The heart belongs 



exclusively to the gills ; and 

 there proceeds from it, not 

 the aorta, but the trunk of 

 those branchial arteries (f), 

 which convey the whole of 

 the blood to the respiratory 

 organs (g, h). This blood, 

 after being there aerated, is 

 collected by the branchial 

 veins (i,) which unite into a 

 single trunk (a), passing down 

 the back, and performing, without any intermediate 

 heart, the office of an aorta ; that is, it divides into 

 innumerable branches, and distributes the blood to 

 every part of the system.* The blood is then 

 reconveyed to the heart by the ordinary veins, 

 which form a large vena cava (c). This vein is 

 generally considerably dilated at its termination, or 

 just before it opens into the auricle; constituting 

 what has been termed a venous sinus. This, then, 

 is followed by the auricle (d) and the ventricle (e); 

 but, besides these cavities, there is also a fourtli (f), 

 formed by a dilatation of the beginning of the 

 branchial artery, and termed the hulbus arteriosus ; 

 contributing, doubtless, to augment the impetus 



* The caudal branch of the aorta is protected by the roots of the 

 inferior spinous processes, joining to form arches througli which it 

 passes; and frequently the artery is contained in a bony channel, 

 formed by the bodies of the vertebrae, which eflcctually secures it 

 from all external pressure. In the Sturgeon even the abdominal 

 aorta is thus protected ; being entirely concealed within this bony 

 canal. 



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