THE CIRCULATING SYSTEM. 



419 



in Birds, we may have a primitively double aortic arch spring- 

 ing from the left ventricle only. Of this primitively double arch 

 it may be the left half only (Fig. 366), which is developed, as 

 in Mammals, or the right half only (Fig. 365), as in Birds. 



When there are two aortic arches, one from each ven- 

 tricle, these two arches may open into each other by a small 

 foramen just outside the heart, as in Crocodiles. 



Thus we see that man's earliest condition is most re- 

 sembled by Fishes, which, however, may have a greater 

 number of arches than are ever developed in him, and, 

 moreover, these arches may all persist simultaneously. Also 

 a variety of other conditions, transitory in man, may be per- 

 manently retained by animals of different kinds. 



FIG. 363. DIAGRAM REPRESENTING 

 THE VESSELS AND AORTIC ARCHES 

 OF A SNAKE, and the changes which 

 the primitive condition (Fig. 358) has 

 undergone. In this and the three 

 following figures the parts left blank 

 are those which abort. 



(After H. Ratkke.) 



a, a, internal carotids ; b, b } external 

 carotids ; c, c, common carotids ; 

 </, </, d, right main aortic arch ; e, 

 vertebral artery ; f,f,f, left aortic 

 arch : g, commencement of the de- 

 scending aorta ; /;, /i, pulmonary 

 artery ; i, remnant of a primitive 

 aortic arch or ductus Botalli here 

 a remnant of the right first aortic 

 arch, No. 5 of Fig. 358. 



FIG. 364. DIAGRAM REPRESENTING 

 THE VESSELS AND AORTIC ARCHES 

 OF A LIZARD, with the changes in- 

 duced on the primitive condition. 

 (After H. Rat /ike.) 



a, a, internal carotids ; b, b, external 

 carotids ; c, c, common carotids : 

 d, d, anastomosis between the in- 

 ternal carotids and the secondary 

 aortic arches ; e, e, right main aoit'.c 

 arch ; f^f, the subc avian arteries 

 (which give off the vertebral, here not 

 r< presented) ; g, commencement of 

 the great dorsal aorta ; //, h, left main 

 aortic arch ; i, i, /', pulmonary arte- 

 ries ; k, k, rudiments of the first 

 (right and left) aortic arches, 

 Nos. 5, 5, of Fig. 358. 



The great arteries of the head and pectoral limbs may 

 diverge from the aortic arch in different combinations in 

 man's own class. 



E E 2 



