ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS. 3 



General Characters.— "l^Q osseaus system of birds is generally 

 remarkable for the lightness and delicacy of its structure, and also 

 for the extent to which many of the component bones have been 

 soldered together by " anchylosis/'* These two characters are 

 induced by the necessity for a low specific gravity in the bones, 

 in order that they may be suitable for an aerial life, coupled 

 with the equal necessity for firm bases of bony support for the 

 powerful muscles necessary for flight. The lightness is attain- 

 ed by the permeation of most of the bones by air cavities, 

 which are in direct communication with the tubes of the lungs, 

 and therefore also with the external air. In some kinds of 

 birds this permeation of the skeleton by air is carried to such 

 an extent that even the toe-bones are hollowed. Other birds, 

 however, such as the Ostrich, which are not formed for flight, 

 have the laro-er bones filled with marrow, as in mammals. 



The requistte strength, which, as we have said, is of equal 

 importance with lightness in the bones of birds, is attained 

 both by anchylosis and by the structure of the bones them- 

 selves which are generally more compact and close-grained 

 than are those of other animals. In consequence of this com- 

 pact structure, the bones of birds are generally distinguished 

 from those of other animals by their whiter color and more 

 porcellanous appearance. The compact structure is in conse- 

 quence of a larger relative proportion of mineral over organic 

 matter which enters into their composition. In such bones as 

 are not permeated by air, this white color and compact struc- 

 ture is not nearly so noticeable, owing to the proportionally 

 o-reater quantity of organic matter which is present. 

 ^ The SkulLr— In order to arrive at a thorough knowledge of 

 the skull of a bird, and its homological relations, it would be 

 necessary, in the first place, to study the development of the 

 skull of foetal birds ; and, secondly, to study in considerable 

 detail the skulls of the mammalia, because since the bones of the 

 skull were mainly named, first of all, from the mammalian 

 skull, we must naturally go back to that, if we wish to arrive 

 at the original meaning of the terras. It would, however, be 

 beyond the scope of the present sketch to attempt such a task. 

 Taking, however, the adult bird-skull, and surveying its general 

 form, we shall be able to arrive at a fair general idea of its struc- 

 ture, though some points in relation to the original compound 

 structure of certain bones must be taken by the reader on 

 trust. The adult bird-skull is generally noticeable for its 

 smooth and rounded cranial portion, and for the great extent 



* Anchylosis, from avKvXo<;, crooked, and so probably a crooked or irregular 

 union. The term is applied to the union by bpny matter of two or more originally 

 distinct bones. 



