THE ARTERIES 



299 



elongates and at the same time other aortic arches arise between the 

 mandibular arteries and the pericardium, these extending dorsally 

 until they meet the backward prolongations of the first, thus form- 

 ing a pair of longitudinal tubes, dorsal to the alimentary tract, the 

 radices aortae (fig. 311). *^-^t^-^»-«>^*-i-/ 



Fig. 319. — Modifications of the aortic arches in diliciciit vertebrates, after Boas; 

 A, primitive scheme; B, dipnoan; C, urodele; Z>, frog; E, snake; f, lizard; G, bird;''£r, 

 mammal, c, ca-liac artery; da, dorsal aorta; dh, ductus Botallii; cc, ic, external and 

 internal carotids; /», pulmonary artery; s, subclavian; tia, ventral aorta. Vessels carrying 

 venous blood black; those with mixed blood shaded; those which disappear ,'dotted 

 outlines. 



The number of pairs of aortic arches varies with the umber ofn 

 gill clefts, the vessels coursing in the septum between the clefts. 

 The number of arches is greatest in the myxinoids, where the number 

 of clefts varies (p. 255); seven or eight in the notidanid sharks; and, 

 as recent investigations tend to show, probably six in the embryos 

 of all other vertebrates. The history of these arches differs greatly 

 in the different classes (fig. 319), there usually being a reduction in 



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