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 ome aortic arch supplies the body with wholly pure blood. his 
aortic arch always arises from the left ventricle, but in Birds it curves 
over the right bronchus, z.e. is a right aortic arch, and in Mammals 
over the left, ze. 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 
