THE CIRCULATION OF THE BLOOD. . 107 
vein and at another an artery. In this respect it resem- 
bles the foetal heart of higher animals. 
In Vertebrates only is the circulating current strictly 
confined to the blood-vessels; in no case does it escape 
into the general cavity of the body. In other respects, 
there is no great advance in the apparatus of the lowest 
Vertebrates over that of the highest Mollusks. The heart 
of Fishes, as in the Oyster, is double, but its position is re- 
versed. Instead of driving arterial blood over the body, 
it receives the returning, or venous, blood, and sends it to 
the gills. Re-collected fron ¢ 
the gills by minute tubes, 
called capillaries, the blood 
is passed into a large artery, 
or aorta, along the back, 
which distributes it by a 
complex system of capil- 
laries among the tissues. 
These capillaries unite with 
the ends of the veins which 
pass the blood into the auri- 
cle of the heart.” : 
In Reptiles generally (as Pe Gales, Police i aes cane Ne 
8 1, g 
Frogs, Snakes, Lizards, and auricle; a, aorta, or main artery. 
Turtles), the heart has three cavities—two auricles and 
one ventricle. The venous blood from the body is re- 
ceived into the right auricle, and the purified blood from 
the Iungs into the left. Both throw their contents into 
the ventricle, which pumps the mixed blood in two diree- 
tions—partly to the lungs, and -partly around the system. 
Circulation is, therefore, incomplete, since the whole eur- 
rent does not pass through the lungs, and three kinds of 
blood are found in the body—arterial, venous, and mixed. 
The ventricle of these Reptiles is partially divided by a 
partition. In the Crocodile, the division is complete, so 
