OSSEOUS SYSTEM OF AVES, 71 



Frigate-bird and Albatross. In this and some other sea-birds, 

 as the Gulls, Awks, and Petrels, the humerus presents a notable 

 ' ectocondyloid ' process on the radial side, near its distal extremity. 



A sesamoid bone is found attached (like the fabella of Mar- 

 supials) to the capsular ligament and the tendons of the extensor 

 muscles, in many of the Raptores, in the Swifts and Hum- 

 ming-birds; it is double in the Guillemots and Penguins, fig. 19, 

 n, n. 



There is a deep depression beneath the tuberosity at the ulnar 

 side of the proximal expansion of the humerus in the Penguins, 

 Ostrich, Awks, and other birds which have no air in that bone ; 

 and it is here that the air-cells are continued into the bone in the 

 majority of the class which have the humerus pneumatic. 



A section of the humerus of the Penguin ^ shows it to be solid ; 

 that of the Awk (Alca) shows a small medullary cavity mth 

 dense and rather thick walls ; that of a pneumatic humerus, as of 

 the Argala,^ e.g., exposes the extreme thinness of the compact 

 wall of a very large cavity, and the loose cancellous lacework at 

 the extremities of the bone : osseous filaments shoot more or less 

 obliquely across different parts of the cavity, serving to strengthen, 

 like tie-beams, the thin walls, and also, being hollow, to convey 

 minute blood-vessels. The proximal half of the bone is divided 

 longitudinally by a loose cancellous partition ; the decussation or 

 anastomoses of the delicate hollow columns give an open reticulate 

 structure to the inner surface of the air-cavity at the two ex- 

 tremities of the bone, which is highly characteristic of the long- 

 bones of birds. 



The radius, fig. 19, />, and ulna, ib. o, are present in all birds, 

 and co-extended between the joints of the elbow and wrist. The 

 antibrachium, so formed, is short where flight is abrogated ; it is 

 but one-third the length of the humerus, e.g., in the Ostrich : it is 

 rather shorter in Guillemots, Divers ( Colymbus), and Gannets 

 {Sula); is about equal in length in Gallince, Psophia, Cariama ; 

 but exceeds the length in most birds of flight. However, in some 

 of the best flyers, e.g. the Albatross where the humerus is ex- 

 tremely long, and in the Smfts and Humming-birds w^here it is 

 very short, the antibrachium is of the same length therewith. 

 Its two bones are so articulated to each other and to the humerus 

 as to be restricted to flexion and extension : scarcely any degree 

 of rotation is admitted, and this adds to the firmness and resistance 

 required for the action of flight. Owing to the obliquity of the 



• XLiv. p. 219, no. 1137. See also, no. 1373, Ostrich. ^ ji_,_ „(j, uos. 



