944 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 
Pulmonary artery, 
Pulmonary veins 
Right auricle 
—— 
Left auricle 
Vena cava - S.ngle ventricle 
Right aorta 
Ventral aorta 
Fig. 133.—HEart or Torroisz. 
system is chiefly that which has returned from the lungs in ar 
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 
Reptiles 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 Reptiles, | 
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. 
