GENERAL CHARACTERISTICS. 



297 



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 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 

 surrounding the pupil and iris, which by their contraction or expansion are con- 



Tmx 



ABC 



UNDER VIEW OF SKULL OF CAPERCAILLIE (A), DUCK (fi), AND RAVEN (c). 



Mxp, maxillo-palatine ; Vo, vomer ; Pa, palatine ; Pt, pterygoid ; f, basipterygoid 

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



sidered to alter the degree of convexity of the aqueous humour and cornea, and 

 thus to render the eye focally adapted to the constantly varying distance of objects 

 during flight. 



External When clothed with its feathers, the bodily conformation of an 



Characters, ordinary bird is that best adapted for cleaving the air with the least 

 possible 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 rapidly 

 decreasing in girth, while the feathers are all directed from the head towards the 

 tail. In those birds in which the neck is not unduly elongated the whole contour 

 is, indeed, spindle-shaped, and may be compared to two cones placed base to base 

 at the thickest part of the body. It is essential to the exigencies of flight that the 

 centre of gravity should be on the lower aspect of the body, as nearly as possible 

 immediately below the points of suspension by the wings ; and, in order to ensure 



