EXTERNAL ANATOMY. NOSTRILS. 
59 
by the narrow membranaceous skin which conceals the 
linear aperture, that the nostrils would never be disco- 
vered by a student unacquainted with this curious form- 
ation : upon raising this lappet, the aperture appears as 
an elongated slit; but which, in the sunbirds(Ciwwjind«) 
is of a semicircular form. The most tumid nostrils are 
those of the pigeons; but in what respect this structure is 
connected with their economy, we know not. There is an 
obvious analogy, however, even between this shape and 
that which is seen in the plovers. In rasorial types the 
aperture is almost lateral — that is, placed on that side of 
the membrane wliich is nearest to the margin of the bill ; 
the membrane itself being unusually thick and strong, 
almost corneous, and often very convex. It is obvious 
that all these are different modes for protecting an organ 
so delicate: but Nature is ever inexhaustible in her vari- 
ations ; and thus we find that in numberless instances she 
accomplishes the same object by the means of bristles, 
or feathers, variously disposed, according to the particu- 
lar habits of the birds. The great majority of insectivo- 
rous birds have setaceous feathers or bristles incurved 
over the nostrils, sufficient to ward off injury, yet leaving 
the orifice, in other respects, exposed to the free action 
of the air. In birds, however, whose habits lead them 
to plunge their bill into substances which might enter 
the apertures, the nostrils are completely covered by 
thick-set featliers, or rigid bristles, reposing flat upon the 
bill. We have already 
adverted to this structure 
in the whole of the wood- 
peckers ; and it is almost 
as general among the 
crows {fig. 26 .), which 
ha ve not only their nostrils. 
but a great part of the base of the bill, protected and even 
covered by stiff bristles. Hence they are enabled, with 
impunity, to perforate the earth and pick holes in the 
ground, during their search for insects, without the least 
particle of dirt getting into their nostrils. The genus 
