AORTIC ARCHES OF BIRDS — GLENNY 533 



radix aortae of the left side then anastomoses, medial to the ductus 

 botalli, to the proximal portion of the pulmonary arch. As has been 

 demonstrated (Glenny, 1943b, 1943d, 1944d), this secondary attach- 

 ment precedes atrophy of the ductus botalli of the left side, and 

 the left radix aortae posterior to the left fourth aortic arch begins 

 to take over the function of the ductus botalli of that side. 



No accurate account of the loss of the ductus caroticus of the 

 right side could be located. It may be assumed that this occurs 

 first as a disconnection at the level of the right systemic arch and 

 perhaps may occur much later than has been suspected. Bhaduri 

 (1939), Finn (1891), Glenny (1944a), Mathew (1944), and Subhap- 

 radha (1944) have reported the persistence of the ductus caroticus on 

 the right side of several birds. Rarely, however, the otherwise func- 

 tionally modified ductus caroticus may retain a short ligamentous 

 connection with the right systemic arch (Glenny, 1944a). It has been 

 inferred that the ductus shawi represents a functionally modified 

 ductus caroticus which comes to serve as the supply to the 

 bronchi, and sends off branches to the syrinx, lung substance, and the 

 oesophagus (Hafferl, 1933). Not altogether satisfactory studies 

 have been made on the exact changes which take place in the ductus 

 caroticus. 



The fact that the right dorsal radix aortae remains as the functional 

 vessel carrying blood to the abdominal aorta does influence the sub- 

 sequent history of the right ductus botalli. This vessel remains 

 functional almost throughout the embryonic life of most birds, and 

 undergoes further atrophy subsequent to hatching. While most 

 orders of birds retain at least a ligamentous vestige of this embryonic 

 vessel, many families show a greater degree of atrophy of this structure 

 than do others. In some species where obliteration is nearly complete 

 there is frequently evidence of its persistence as a linea botalli along 

 the ventral face of the dorsal radix, with which structure it may fuse. 



With the atrophy of the right ductus botalli, the left radix aortae 

 begins to atrophy. Tbis process continues in most birds until only 

 a small ligamentum aortae remains as the vestige of this once promi- 

 nent vessel. Rarely, the left radix aortae may remain as a function- 

 ally modified vessel (Glenny, 1939) or, more frequently, with a 

 short lumen. In general it may be stated that almost without excep- 

 tion extremely careful examination of the adult bird wiU reveal a 

 minute ligamentous vestige of the left radix aortae. The difficulty 

 encountered in determining its presence arises from the fact that the 

 ligament may become so much reduced that it is difl[icult to differ- 

 entiate it from the surrounding fascia, and in smaller birds it is stiU 

 more diflScult to find. 



When the right ligamentum botalli is much reduced, its distal 

 attachment to the radix aortae may be determined by the presence 



