282 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
the division of the ventricle is completed by the extension of the 
septum to the anterior end, but there is an opening (foramen 
Pannizz) between the two sides of the aortic trunk, so that some 
admixture of arterial and venous blood can occur. In the birds 
and mammals (fig. 287, F) there is complete internal separation 
of the two sides of the heart, though externally it shows but slight 
signs of the division. Asa result of this division blood must pass twice 
through the heart (once through the venous, once through the arterial 
half) in order to make a complete circuit of the body. Venous blood 
enters the right atrium, passes to the right ventricle, by which it is 
forced to the lungs (pulmonary or respiratory circulation). Re- 
turning to the heart by the pulmonary veins, it passes through the left 
atrium and ventricle and thence through the systemic circulation, by 
which all parts of the body are supplied. Details of the modifications 
of the heart in the different classes of vertebrates are given at the end of 
‘this chapter. 
Aortic ARCHES. 
As was said above, the typical number of aortic arches is six pairs, 
this number being but rarely exceeded. In all groups except cyclos- 
tomes and fishes they undergo considerable modification, and in the 
fishes they are frequently more or less reduced in correlation with the 
reduction of the gills (p. 238). The modifications may be outlined as 
they occur in the successive pairs of arches. . 
In many fishes and all tetrapoda the first arch on either side dis- 
appears beyond the point where the external carotid arises, while, 
correlated with the reduction of the spiracular gill, the second pair of 
arches is partially or completely lost in the adult. The third pair is 
always persistent and through them flows the blood for the internal 
carotids and, in the fishes, gymnophiona and a few urodeles (fig. 280, 
C) and reptiles, (EZ) blood for the radices aorte as well. In all other 
tetrapoda the radix disappears between the third and fourth arches (fig. 
280, D) and consequently here the third arch is purely carotid in char- 
acter. When this occurs the portion of the ventral aorta between the 
third and fourth arches carries blood for the carotids alone and hence 
forms a common carotid trunk, usually divided into right and left 
common carotid arteries. 
The fourth pair of arches are the systemic trunks in all tetrapoda, 
