292 THE SIXTH DAY. [CHAP. 



The first pair of arches, before it entirely disappears, 

 sends off on each side two branches towards the head. 

 Of these, one forms the direct continuation of the bulbus 

 arteriosus in a straight line from the point where the 

 first aortic arch leaves it ; primarily distributed to the 

 tongue and inferior maxillary region, it becomes the 

 external carotid (Fig. 92, E.G.A.). The other, starting 

 from the point where the aortic arch of each side joins 

 its fellow, dorsal to the alimentary canal, to form the 

 dorsal aorta, is primarily distributed to the brain, and 

 becomes the internal carotid (Fig. 92, I.G.A.). 



When the first arch disappears, the external carotid 

 arteries still remain as the anterior continuations of the 

 bulbus arteriosus. And since the dorsal trunks uniting 

 the distal ends of the first and second arches do not 

 become obliterated at the time when the first pair of 

 arches disappears, the internal carotids remain as 

 branches springing from the distal ends of the second 

 pair of arches ; they are supplied with blood from that 

 pair, the stream in which flows chiefly towards the head 

 instead of backwards towards the dorsal aorta, as is the 

 case with the succeeding arches. When the second 

 pair of arches is obliterated, the connecting branch with 

 the next arch is again left, and thus the internal carotids 

 appear as branches from the distal ends of the third 

 pair of arches. 



On the third day the dorsal aorta does not for any 

 distance remain single in its backward course along the 

 body, but soon divides into two trunks which run one 

 on either side of the middle line of the body. These 

 two trunks, as development proceeds, gradually unite 

 along their whole length, and there is thus formed a 



