EXTERNAL FEATURES 



393 



have a wide tread, the first being directed backwards and the 

 other three forwards ; the fifth is wanting. The front or facial 

 portion of the head is drawn out into a beak covered with horny 

 skin. At its base above is a swollen, featherless patch of skin, 

 the cere. The nostrils lie below the cere, the eyes behind it at 

 the sides of the head, and the ear openings below and behind the 



Fig. 308. — A plucked pigeon, seen in dorsal view. 



apt., Apteria cr., cere; ear; na., nostril; o.g.p., papilla on which the oil gland opens; 



ptl., pterylae ; px., thumb. 



eyes, covered by feathers. There are three movable eyelids and 

 the drum of the ear is at the bottom of a tube, but there is no 

 ear flap. There is a single cloacal opening, as a transverse slit 

 below the tail, and above the tail is a knob on which opens 

 the oil gland, the secretion of which is used in preening the 

 feathers. 



FEATHERS 



The most conspicuous feathers, and the most important for 

 the bird, are those of the wing. They are grouped, and partly 



