GENERA L CHAR A CTERISTICS. 
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- 
UNDER VIEW OF SKULL OF CAPERCAILLIE (a), DUCK (B), AND RAVEN (c). 
Mxp, maxillo-palatine ; Vo, vomer ; Pa, palatine ; Pt, pterygoid ; f, lmsipterygoicl 
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 
