VASCULAR SYSTEM 505 



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 arteriosus in other Fishes. 

 In Vertebrates 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 heart has two auricles and an 

 incompletely 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. 



In Birds and Mammals the heart has two auricles and two ventricles, 

 and one aortic arch supplies the body with wholly pure blood. This 

 aortic arch always arises from the left ventricle, but in Birds it curves 

 over the right bronchus, i.e. is a right aortic arch, and in Mammals 

 over the left, i.e. is a left aortic arch. Impure blood from the body 

 enters the right auricle, passes into the right ventricle, is driven to the 

 lungs, returns purified to the left auricle, enters the left ventricle, and is . 

 driven to the body. 



The arterial system of a fish consists of a ventral aorta continued 

 forwards from the heart, of a number of afferent vessels diffusing the 

 impure blood on the gills, and of efferent vessels collecting the purified 

 blood into a dorsal aorta. 



In the embryo of higher Vertebrates the same arrangement persists, 

 though there are no gills beyond Amphibians. From a ventral arterial 

 stem arches arise, which are connected so as to form the roots of the 



