244 CIRCULATION IN REPTILES. 



blood is not complete ; and whilst the Wood 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 ^^jtJ^f^^WjS^^S*^** Pulmonary artery 



fll /^Jsi40fe. fflxmPulmonary vein 

 Pulmonary vein ] 



Right auricle- 



Left auricle 



Single ventricle 



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 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. 



