338 HISTORY OF THE HUMAN BODY 



ship to the stapes. How it comes to bear the three important 

 branches of the later external carotid has been already seen. 



Between stages h and i a second important change has 

 been inaugurated in the reduction of that part of the dorsal 

 aorta which connects arches III and IV. This finally effects 

 a complete separation of the two arches in this place, and 

 causes the third arch to become a common carotid artery (cc) 

 which divides into two branches, an external carotid (ce) 

 which was formerly the anterior part of the ventral aorta plus 

 the ventral stumps of arches I and II, and an internal carotid 

 (a), the main third arch plus the original arteria carotis 

 cerebralis. 



One more change in relationship is to be effected, and that 

 is inaugurated through the growth of another anastomotic 

 branch (n in stage i) which enters the side of the mandibularis 

 (or, perhaps, the continuation of the stapedialis) and forms a 

 complete circuit, as in stage k. From this point on the history 

 differs in the rat and in Man, as is indicated by the two 

 arrows, with their respective designations. In the rat the cir- 

 cuit breaks at the point between the infra-orbital and the man- 

 dibular, and in Man at a point above the supra-orbital. The 

 two results of these are seen in diagrams / and n, which rep- 

 resent the adult condition of this detail in the rat and in Man, 

 respectively. In the former (/) the stapedial artery, a branch 

 of the internal carotid, bears both supra- and infra-orbital 

 arteries, while the external carotid becomes continued mainly 

 into the mandibular. In the latter the external carotid bears 

 all three of the branches in question, while the stapedial ar- 

 tery, being of no further use, disappears, and leaves in the 

 stapes the hole through which it formerly ran, thus account- 

 ing for the particularly curious shape of this little bone, which 

 attracted the attention of the early anatomists, but for which 

 they had no explanation. In considering the details of the de- 

 velopment of any part of the circulatory system, the process 

 is seen to be a metamorphosis, correlated with the changes 

 in the parts supplied by the blood-vessels under consideration. 

 Such a metamorphosis is like the changes in the roads and 



