GENERA L CHA RA CTERIS TICS 



1469 



so as to form a bridge in front of the vomer, the construction is termed desmogna- 

 thous (bridged palate). In a third modification, as exemplified in the raven (B) 

 and all other living passerine birds, the maxillo-palatines, although extending be- 

 neath 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 csgithognathous (passerine-palate). Various minor modifications of these 

 three types exist, but a little practice will enable the student to determine to which 

 of the three any given skull conforms. A fourth modification, met with only among 



fmx 



UNDER VIEW OF SKUI.I. OF CAPERCAILZIE (A), DUCK (B), AND RAVEN (C). 



Mxp. maxillo-palatine ; Vo. vomer ; Pa. palatine ; Ft. pterygoid ; f- basipterygoid facet. 



(From Proc. Zool. Soc., 1867. After Huxley.) 



the ostrich-like birds and the South- American tinamus, need not be referred to till 

 a later chapter. 



Before leaving the subject of the skull, it may be mentioned that the outer coat 

 or ' ' white ' ' of the eye of a bird contains a movable ring of overlapping bones sur- 

 rounding the pupil and iris, which by their contraction or expansion are considered 

 to alter the degree of convexity of the aqueous humor and cornea, and thus to render 

 the eye focally adapted to the constantly varying distance of objects during flight. 



When clothed with its feathers, the bodily conformation of an ordi- 

 External Char- i j ,1 .1 j i r < -,11 



. . nary bird is that best adapted for cleaving the air with the least pos- 

 sible resistance; the head being more or less sharpened, the body 

 gradually swelling to a point some distance in advance of the middle, and then as 



