THE HEART AND ITS VESSELS. 



277 



FIG. 221. DIAGRAM SHOWING THE TRANSFORMATIONS OF THE AORTIC ARCHES 

 A, IN A LIZARD ; B, IN A SNAKE ; C, IN A BIRD ; AND D, IN A MAMMAL. 

 (After Rathke.) (Seen from below.) 



A . a, internal, and b, external carotid ; c, common carotid ; d, ductus Botalli between 

 the third and fourth arches ; e, right aortic arch ; /, subclaviaii ; g, dorsal aorta ; 

 h, left aortic arch ; i, pulmonary artery ; k, rudiment of the ductus Botalli 

 between the pulmonary artery and the aorta. 



B. a, internal, and b, external carotid ; c, common carotid ; d, right aortic arch ; 

 e, vertebral artery ; /, left aortic arch ; h, pulmonary artery ; i, ductus Botalli 

 of the latter. 



C. a, internal, and b, external carotid ; c, common carotid ; d, base of the aorta ; e, 

 fourth arch of the right side (root of the aorta) ; /, right subclavian ; g, dorsal 

 aorta ; h, left subclavian (fourth arch of the left side) ; i, pulmonary artery ; k 

 and I, right and left ductus Botalli of the pulmonary arteries. 



D. a, internal, and 5, external carotid ; c, common carotid ; d, base of the aorta ; e, 

 fourth arch of the left side (aortic root) ; /, dorsal aorta ; g, left vertebral artery ; 

 h, left subclavian ; i, right subclavian (fourth arch of the right side) ; k, right 

 vertebral artery ; I, continuation of the left subclavian ; m, pulmonary artery ; 

 n, ductus Botalli of the latter. 



THE HEART AND ITS VESSELS. 



Fishes. While the heart of Amphioxus is not specially 

 differentiated, as it is in the Vertebrata, that of Fishes is well 

 developed, and is situated in the anterior part of the body-cavity, 

 close behind the head. It is always formed on the same type 

 as that described on p. 269. 1 In it may be distinguished a 



There is no truncus arteriosus in Cyclostomi. 



