VI.] THE ARTERIAL ARCHES. 169 



pair of arches we may consider that there are in all four 

 pairs of arches. When the first and second arches are 

 obliterated, it is only the central portion of each arch on 

 either side which absolutely disappears. The ventral portion 

 connected with the bulbus arteriosus, and the dorsal portion 

 which joins the dorsal aorta, both remain, and are both 

 carried straight forward towards the head. The ventral 

 portions of both first and second arches unite on each side to 

 form a branch, the external carotid (Fig. 52, E. CA), which 

 runs straight up from the bulbus arteriosus to the head. 



FIG. 52. 



JE.C+. 



T.CA 



STATE OF ARTERIAL CIRCULATION ON THE FIFTH OR SIXTH DAT. 



E. CA. external carotid. /. CA. internal carotid. AO. dorsal aorta, wf. A. 

 arteries to the Wolffian bodies. Ver. A. arteries given off between each of 

 the vertebrae. Of. A, omphalo-mesaraic artery. UA. umbilical artery. 

 I A. iliac artery. 



In the same way the dorsal portions form a branch, the 

 internal carotid, which takes its origin from the dorsal or far 

 end of the third arch. 



22. In the venous system important changes also occur. 



As the liver in the course of its formation wraps round 

 the common trunk of the omphalo-mesaraic veins, or meatus 

 venosus, it may be said to divide that vessel into two parts : 



