534 PROCEEDINGS OF THE NATIONAL MUSEUM vol. io4 



of a small ligamentous button on the ventral face of the right radix. 



The systemic arches in bu-ds are pau-ed structures only dming 

 early embryonic stages. Biswas (1946), however, reported the 

 anomalous occurrence of both left and right systemic arches in a 

 specimen of Ploceus philippinus philippinus. 



Normally, atrophy of the left systemic arch follows shortly after 

 the disconnection of the ductus caroticus from the posterior portion of 

 the dorsal radices aortae. This results in the retention of the right 

 systemic arch as a functional vessel which then passes diagonally 

 lateral and dorsad to join the remaining functional right dorsal radLx 

 aortae which then passes diagonally toward the midline to the point 

 of union with its complementary vessel of the left side. The latter 

 vessel is usually found in the adult as the ligamentum aortae. The 

 functional radix then forms a connection with the abdominal aorta. 



In the respect that birds present but one of the pair of systemic 

 arches, they differ from mammals. On the other hand, the right dor- 

 sal radix aortae in birds and the left dorsal radix aortae in mammals 

 are the sole functional vessels which are responsible for the distribu- 

 tion of the blood to the abdominal viscera and posterior appendages. 



As is well known, the ventral or proximal portion of the embryonic 

 sixth aortic arch remains as the functional portion of this embryonic 

 vessel which, along with the embryonic pulmonary artery that joins 

 it, comes to serve as the definitive pulmonary artery of the adult. 



The left ductus botalli usually undergoes atrophy shortly after the 

 complete atrophy of the left systemic arch, by which time the left 

 radix forms an anastomosis with the pulmonary arch proximal to the 

 normal dorsal (ductus botalli-radix aortae) connection. As a result 

 of this secondary connection, the left dorsal radix aortae serves the 

 same function as the ductus botalli (Glenny, 1943d, 1944d). This is 

 not the case in anomalous retention of the left systemic arch as re- 

 ported by Biswas (1946). In this rather singular case, the distal 

 portion of the left radix atrophied and the connection was maintained 

 by way of the left systemic arch, and the left ligamentum botaUi 

 remained as the vestige of the embryonic vessel, whereas in most cases 

 the left ligamentum botaUi completely atrophies, or at best becomes 

 fused with the left radix aortae either prior to or at the same time as 

 the radix undergoes atrophy. 



In instances of functional modification of the left radix aortae, the 

 left ligamentum botalli may or may not be completely lost; but this 

 is extremely difficult to ascertain since so few species or individuals 

 may retain a functional left radix aortae and atrophy of the ligamen- 

 tum botaUi has usually progressed to such a stage that determination 

 of its presence is difficult. 



The distal portion of the right sixth aortic arch undergoes atrophy 

 and becomes the ligamentum botalli or it may rarely maintain a small 

 lumen. In such instances where it does not appear to be present it 



