296 



BIRDS. 



Skull. 



LOWER END OF THE LEFT TIBIA OF A 

 CRANE (A), AND A YOUNG OSTRICH (b). 



which pass beneath. Such pulleys enable the fleshy portions of the muscles to be 

 placed high up in the limb, and thus cause the centre of gravity of the body to 

 be near the wings, an arrangement essential for flight. In addition to the three 

 toes articulating with the lower end of the cannon-bone, most birds have another 



toe, corresponding to the first or great toe of 

 B the human foot, of which the metacarpal is 



loosely attached to a facet on the inner edge 

 of the hinder surface of the cannon-bone — as 

 shown in the figure of the cannon-bone of a 

 buzzard in our fourth volume. No bird has 

 any trace of the fifth toe. The number of 

 joints in each toe, in place of not exceeding 

 three as in ordinary mammals, increases regu- 

 larly from the first to the fourth toe. 



As the structure of the base 

 of the skull is of some import- 

 ance in classification, a few words are neces- 

 sary on this point. In the first place, the 

 skull of a bird is characterised by the great size 

 of the sockets for the eyes, which are separated from one another merely by a 

 thin bony partition. The aperture for the nostrils (immediately below Na in the 

 figure on p. 292) may be either short and rounded, when the skull is said to be 

 holorhinal (as in that figure) ; or they may form elongated slits, as in a pigeon, 

 when the condition is termed schizorhinal. In all Birds most of the component 

 bones of the skull are completely united together, without any trace of the original 

 lines of division, in the adult state ; and in ornithology it is usual to apply the 

 terms upper and lower mandible to the two parts of the beak. 



With regard to the bones of the palate, the introduction of a number of 

 technical terms is unavoidable. In the middle of the hinder part of the lower 

 surface of a bird's skull can be seen a pointed rod of bone, known as the 

 *l>henoidal rostrum, which may carry, as in (A) of the figure, a pair of basipterygoid 

 facets (j-). In advance of this is a single or double bone, termed the vomer (Vo). 

 On the two sides of this central axis are two pairs of slender bones, of which the 

 hinder are termed pterygoids (Pt), and articulate with the basipterygoid processes 

 when present; while the front pair are named 'palatines (PI). From the sides of 

 the upper jaw or maxilla? (Mx), are given off two maxillo -palatine processes (Mxp), 

 projecting in the middle line towards the vomer. Now when the vomer, as in the 

 fowl and capercaillie (A) is pointed in front, while the maxillo-palatines remain 

 separate both from it and from one another, the skull is said to be schizognathous 

 (cleft palate). When, on the other hand, as in the duck (B), the maxillo-palatines 

 unite in the middle line, so as to form a bridge in front of the vomer, the construc- 

 tion is termed desmognathous (bridged palate). In a third modification, as 

 exemplified in the raven (B) and all other living passerine birds, the maxillo- 

 palatines, although extending beneath the vomer, do not unite either with that 

 bone or with one another, while the vomer itself is expanded and abruptly 

 truncated in front ; this arrangement being termed cegithognathous (passerine- 



