VI 



AETERIAL SYSTEM 



395 



is comparatively small, (he aortic arch can be traced throughout as a 

 distinct channel ; while in the Amniota, where the walls of the gill- 

 clefts have completely lost their respiratory function, tin- respiratory 

 network never develops at all. 



an 





UC. C. 



r / 



IT / n 



Fi<;. ISO. Scheme of aortic arches and their derivatives as seen from the ventral side. 

 The parts of the original scheme which disappear during development are shown in 

 pale tone. 



A, complete unmodified series of aortic arches; 15, arrangement in a Urodele ; C, in a Reptile; 

 D, in a Mammal. .-1, dorsal aorta; a.r, aortic root; on, nnastomotic vessel; <-.r, common carotid: 

 il.1l, ductof Hotallus ; /... dorsal (internal) carotid ; i, innominate artery ; /.;>, left pulmonary artery ; 

 Z.s, left systemic; />, pulmonary; /.;>, right pulmonary; .s', systemic aorta; >-, subclavian artery; 

 v.'i, ventral aorta; <-..-. v.-ntral (external) carotid ; 1, II, etc., aortic arches. 



The longitudinal vessels with which the aortic arches are con- 

 nected are prolonged forwards as the carotid arteries which supply 

 i .In- head with blood. The ventral aorta is prolonged forwards as the 

 ventral, or external, carotid while the prolongation forwards of the 

 aortic root forms the dorsal, or internal, carotid. Of the alternative 



