244 
CIRCULATION IN REPTILES. 
blood is not complete; and whilst the blood transmitted to 
the lungs is chiefly that which has returned from the systemic 
veins, the blood which enters the aorta for the supply of the 
Pulmonary artery^ 
Pulmonary vein** 
Right auricle— 
Vena cava— 
Right aorta— 
If-- -— Ventral aorta 
Fig. 133.— Heart of Tortoise. 
system is chiefly that which has returned from the lungs in an 
arterialized state. Hence such animals have a circulation 
which approaches very closely to that of Mammals and Birds ; 
and it is among them that we find the greatest vigour and 
activity in this generally inert and sluggish class. 
285. The general arrangement of the blood-vessels in 
Eeptiles is shown in fig. 134. It is seen that the aorta, 
soon after its origin, divides into three arches on either side ; 
and that these, after sending off branches to the head and to 
the lungs, reunite into a single trunk, which corresponds 
exactly with the aorta of the higher animals. These arches 
are in fact the remains of a set of vessels, which will be 
found to be of the highest importance in Fishes, being there 
subservient to the aeration of the blood : in the true Eeptiles, 
however, they are never concerned in this function, but they 
still remain, as if to show the unity of the plan on which 
this apparatus is formed. Precisely the same arrangement of 
the vessels may be seen in Birds and Mammalia, at an early 
stage of their development; but it afterwards undergoes* 
considerable changes, by the obliteration of several of the 
arches ; for of the four pairs which may be seen at one period, 
a single branch only remains on either side; and one of these 
becomes the permanent arch of the aorta, whilst the other 
becomes the permanent pulmonary artery. 
Pulmonary artery 
Pulmonary vein 
Left auricle 
Sngle ventricle 
