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adult state have hollow humeri, the cavity of these bones is filled with 

 thin marrow ; and probably it is not till some time after the wings 

 have been used, that the air-hole is formed by the absorption of the 

 membranous covering. 



In my last paper I stated that in one Swift (C. apus) the humeri 

 were filled with marrow, and in two others they were hollow. I 

 have only been able to obtain one other specimen, and in this the 

 humerus was hollow ; so that the first described was probably a 

 young bird : and the same remark will apply to the Goat-sucker 

 (Caprimulgus), which I now find has in the adult state a hollow 

 humerus. 



The hollow bones are strengthened at their extremities by cross 

 and transverse beams ; but notwithstanding the assertion of many, 

 that a hollow bone is stronger than one containing marrow, I believe 

 these bones to be considerably weaker : they are readily splintered 

 by shot, as all who are accustomed to shoot a variety of birds are 

 aware, from the number of broken wings among the accipitrine and 

 gallinaceous birds. Some humeri containing marrow are of great 

 strength and thickness, and very difficult to break ; indeed a shot 

 may pass through them without splintering the bone. The humerus 

 of the Great Northern Diver (C. glacialis) on the table, is as thick 

 and heavy as that of most quadrupeds : the cavity for the marrow 

 is very small, and the parietes of the cylinder measure 3 lines. In 

 addition to its great weight, it is 3 inches in length, forming a re- 

 markable contrast with the Swift (C. apus), the length of which is 

 only 4 lines. 



Before closing this division of my paper, I may mention that most 

 writers, in describing the humerus of a bird, speak of the air-hole as 

 if generally present ; but I believe in the majority of birds it is not 

 to be found in this or any other bone. 



The manner in which air is supplied, and the muscular apparatus 

 connected with the humeral and femoral apertures, will, as I have 

 said before, be given in another communication. I may merely ob- 

 serve here, that a prolongation of the thoracic air-sac is continued 

 over the joint, so as to allow of the most perfect mobility. The 

 thigh-bone, when hollow, is supplied with air in a like manner. 



The flight of birds in relation to the presence of air in the bones. 

 — Notwithstanding the assertion before quoted, " that in the diurnal 

 birds of prey, as in almost all other birds of flight, the femur is 

 filled with air," it will be found, on reference to the table, that 

 scarcely one bird of flight has a hollow femur, and that the great 

 majority of the British birds of passage have no air in the bones, 

 judging from those examined ; thus of the twenty-one birds of 

 passage named in the tables, only five, the Turtle-Dove, Swift, 

 Goat-sucker, Cuckoo and Bean Goose had hollow humeri, but these 

 had no air in the femur ; the remaining sixteen had marrow in all 

 the bones of the limbs. It will be seen also that many birds of 

 short flight, as the Tits, Woodpeckers, and others, have hollow 

 humeri. 



The presence of air too in the humerus does not appear to in- 



