74 F. W. THYNG 



this artery represents a persistent portion of the dorsal part of the 

 second aortic arch, and an intermediate segm.ent of the first arch; 

 the two being united by a longitudinal anastomosis. In certain 

 mammals the stapedial artery persists throughout life, but in man 

 an anastomosis between the external carotid artery and the man- 

 dibular ramus of the stapedial results in the formation of the adult 

 internal maxillary artery to which the stapedial transfers its 

 branches. In this embryo the anastomosis is yet to form; there 

 can be no doubt, however, that it will join the lateral branch of the 

 ventral division of the stapedial between the auriculo-temporal 

 nerve and the branch of communication between the lateral and 

 medial branches. As a result of such an anastomosis the lateral 

 branch of the ventral division, together with the dorsal division 

 (supraorbital of Tandler), would become the middle meningeal. 

 The middle meningeal and inferior alveolar would then spring 

 from the internal maxillary. The original main trunk of the 

 stapedial would persist in part as the carotico-tympanic branch 

 of the internal carotid and the superior tym.panic branch of the 

 middle meningeal. It is extremely probable that the medial 

 branch of the original ventral division of the stapedial is repre- 

 sented in the adult by the accessory meningeal branch of the 

 internal maxillary. 



The right ventral aorta (plate 1) takes a cephalic direction 

 from the truncus aorticus. It soon gives off a short dorsal 

 branch, the fourth aortic arch, which becomes part of the right 

 subclavian. The portion of the right ventral aorta proximal to 

 the fourth arch becomes the innominate artery {A. anon.), the 

 smaller distal continuation represents the right common carotid. 

 The relations of the latter are essentially the same as those just 

 given for the corresponding artery on the left. A branch, how- 

 ever, from the proximal part of the right internal carotid, accom- 

 panies the hypoglossal nerve. This, the hypoglossus artery 

 (A.hyp.d., plate 1) has been noted in the human embryo by Zim- 

 mennan ('89), Tandler ('02) and Ingalls ('07), and in the 

 rabbit by Hochstetter ('90). The hypoglossus artery of the 

 left side had disappeared in this embryo. The hypoglossus 

 arteries represent the second pair of primitive intersegmental 



