218 THE SIXTH DAY. [CHAP. 



of the left side, and primarily takes its origin from the connecting branch 

 between the fourth and fifth arches. Its mode of development thus nearly 

 agrees with that of the right subclavian of mammals. In support of this view 

 of the development of the left subclavian, is the fact that in some birds there 

 is, between it and the dorsal aorta, a fibrous connection which is the remains of 

 the vessel which originally carried the blood from this fourth arch to the dorsal 

 aorta. On the left side therefore, as can easily be understood by reference to 

 the diagram, the subclavian without further alteration remains as the continu- 

 ation of the left innominate, L.I.N. 



In consequence of these changes the condition of the 

 aortic arches during the latter days of incubation, before 

 respiration by the lungs has commenced, is as follows 

 (Fig. 66). 



The first and second arches are completely obliterated. 

 The third arch on each side is continued at its distal end as 

 the internal carotid, I.Ca, the connection between it and 

 the fourth arch having become entirely obliterated. From 

 its proximal end as the direct continuation of the trunk which 

 originally supplied the first and second arches the external 

 carotid, E.CA., is given off. Each pair of carotids arises 

 therefore from a common trunk — the common carotid (C. 

 C.A.). Each of these trunks gives off near its proximal 

 end a branch, the vertebral artery (V.A.), which joins at 

 its distal end a branch from the external carotid. 



The common carotid on the risdit side comes off from the 



o 



fourth arch of the right side (the arch of the dorsal aorta), 

 and is not as yet connected with the right subclavian, B.sc. 



The common carotid of the left side comes off from the 

 fourth arch of the left side ; but since this arch becomes the 

 left subclavian, L.sc. (the connection between the fourth and 

 fifth left arches being obliterated), the portion of the trunk 

 (L.I.N.) between the fourth arch and the bulbus arteriosus (or 

 as it must now be called the common aortic root) is called 

 the left innominate artery. 



The fourth arch of the right side forms the commence- 

 ment of the great dorsal aorta, and gives off the right 

 subclavian (R. sc.) just before it is joined by the fifth arch. 



The fifth arch of each side gives off branches P. A. to the 

 lungs ; their distal continuations BG.b., L. C.b., by which they 

 are connected with the systemic circulation, though much 

 reduced, are not obliterated. 



The final changes undergone by the arterial system 

 after the commencement of the pulmonary respiration consist 



