VASCULAR SYSTEM 



561 



or by two auricles ; thence the blood passes into the 

 muscular ventricle or ventricles, and is driven outwards. 

 Except in adult Birds and Mammals, the veins from the 

 body enter the auricle (or the right auricle if there are two) 

 by a porch known as the sinus venosus. In Fishes (except 

 Teleosteans) and in Amphibians 

 the blood passes from the ventricle 

 into a valved conus arteriosus, 

 which seems to be a continuation 

 of the ventricle. In Teleosteans 

 there is a superficially similar 

 structure, but without valves and 

 non-contractile, and apparently 

 developed from the aorta, not 

 from the ventricle ; it is called 

 the bulbus arteriosus, and may 

 occur along with the conus arteri- 

 osus in other Fishes. In Verte- 

 brates higher than Amphibians 

 there is no distinct conus. 



In Cyclostomata, and in all Fishes 

 except Dipnoi, the heart has one auricle 

 and one ventricle, and contains only 

 impure blood, which it receives from the 

 body and drives to the gills, whence 

 purified it flows to the body. 



In Dipnoi the heart is incipiently three- 

 chambered. 



In Amphibians the heart has two 

 auricles and a ventricle. The right 

 auricle always receives venous or impure 

 blood from the body, the left always 

 receives arterial or pure blood from 

 the lungs. The single ventricle of the 

 amphibian heart drives the blood to the 

 body and to the lungs. 



In all Reptiles, except Crocodilia, the 



Fig. 319. — Diagram show- 

 ing the valves (VA.) 

 common in veins, which 

 prevent the blood from 

 flowing backwards if 

 there should be some 

 obstacle to the normal 

 flow. Valves usually 

 occur in pairs. Their 

 ivee margin is towards 

 the heart. They are riot 

 found in arteries. 



heart has two auricles and an incom- 

 pletely divided ventricle. The partition in the ventricle secures that 

 much of the venous blood is sent to the lungs ; indeed, the heart, 

 though possessing only three chambers, works almost as if it had four. 

 In Crocodilia there are two auricles and two ventricles. But the 

 dorsal aorta, which supplies the posterior parts of the body, is formed 

 from the union of two aortic arches, one from each ventricle. Therefore 

 it contains mixed blood. 



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