BIRDS. 231 
urally be assumed that under the vigorous beating of the 
wing they would form little, if any, resistance. There is, 
however, a special arrangement, shown in Fig. 273, by 
which each barb is locked with its fellow by little second- 
ary branches called barbules, that are generally serrated 
and provided with hooks, and in this way a strong surface 
is presented to the wind. Plumes, as those 
ae og of the ostrich, are 
ee ET! feathers without 
barbules. Down 
is feathers in 
which the barbs 
are extremely 
soft and free. 
The feathers 
shed water by 
being oiled with 
a secretion the 
birds take from 
an oil-gland near 
the tail. After 
the reproductive 
Fic, 272.—Parts 
Fic. 273.—Barb from a 
of a feather. 
season, birds gen- 
goose-quill, showing 
x, quill; 2, erally moult or the hooklets highly 
shaft; 3,3, shed their feath- magnified. 
vane or barbs; 
4 accessory OT Some birds, 
plume. as the ptarmigan, do this two or three 
times a year. 
Senses.—The eyes of some birds are remarkably pierc- 
ing, and their vision extremely acute. All possess a third 
eyelid or nictitating membrane (Fig. 270, 2) that covers and 
protects the ball of the eye, so that eagles can gaze directly 
at the sun without blinking. The eyes of these birds and 
their allies are also provided with a ring of hard plates that 
forms an apparatus by which the bird can adjust its sight 
to objects near at hand or at a distance. The penguin, 
