8 
pectinate body, which tills with blood and occupies the space vacated 
bv the lens; the cornea becomes more convex and the bird exerts 
his best efforts to fix a minute near object. Does he desire to see 
distinctly in the far distance, the converse is true; Crampton^s 
muscle relaxes, the eyeball becomes more globular, the tense corneal 
cone becomes less prominent, the lens recedes and is less globular, 
the anterior chamber deepens, the pecten is flaccid, and the antero¬ 
posterior diameter sensibly diminishes. Thus the important func¬ 
tion of accommodation in birds is a much more complex and exten¬ 
sive performance than in man. In this way the bird is able, as no 
other animal can, to convert his organ of vision, as Beebe remarks, 
“from a microscope to a telescope” in a fraction of a second—to 
see small objects a quarter of a mile away and to pick from the 
ground seeds so small that one would need a lens to distinguish them 
from surrounding grains of dust. 
The crystalline lens in most birds has, as just remarked, a globu¬ 
lar form. It is comparatively larger than the human crystalline, 
and in the nocturnal birds of prey has such a markedly convex 
anterior surface as to be almost round. In birds requiring chiefly 
good distant vision the round lens is a compound structure built 
up of concentric layers surrounded by a ring of radial fibers widest 
at the equator. This disposition of the lens fibers assists in pro¬ 
ducing the lenticular changes required by the active accommodation 
called for in birds. The globular lens, again, corresponds to the 
prominent cornea and deep anterior chamber. In water fowl the 
lens is flatter in front, i. e., more plano-convex in shape, and the 
cornea is not so conical. 
In passing, it may be remarked that the postmortem appearances 
of the eyeball, especially of the cornea, are somewhat misleading, 
as there is always more or less rounding of the globe through sink¬ 
ing and retraction of the cornea shortly after the death of the ani¬ 
mal. .For this reason, also, microscopic sections of the globe gen¬ 
erally fail to show the tubular-oval shape that the external eye gen¬ 
erally maintains during life. 
The Pecten . — This is the most peculiar organ in the whole ocu¬ 
lar apparatus of birds. From the optic disc of every bird, 4 of their 
4 The statement that the Kiwi, or Apteryx, is without a pecten (vide e - 
Bernd, Die Entwicklung des Tectens, Inaug. Diss. Bonn 1905 n S'! it * 
by Lindsay Johnson. The same error is repeated in most P t«t wt. p ov ? a 
Claus's Lehrbuch der Zoologie, 5th edition, 1801, p 845. text-books, as m 
