EXTERNAL APERTURES. 433 



The secondary quills are those attached 

 to the ulna, 

 ii. The rectrices, or tail- quills, have the two halves 

 of the vane about equal in size. They are 

 used by the bird for steering during flight. 



There are twelve in the wild rock-pigeon, 

 but may be more in domestic breeds. 



b. The coverts are the smaller feathers which cover 



the bases of the quills of the wings and tail. 



c. The contour feathers are the short soft feathers 



covering the body generally. The barbules are 

 less perfectly developed than in the quill- 

 feathers, so that the barbs easily separate from 

 each other, especially at the base of the vane. 



d. The filoplumes are the minute rudimentary feathers 



left in the skin after the bird has been plucked. 

 Each consists of a hair-like stem, with a very 

 rudimentary vane at its apex. 



3. Arrangement of the feathers. 



The feathers are attached to certain definite areas 

 of the skin, known as pterylse, the intervening tracts 

 or apteria being devoid of feathers. This arrange- 

 ment is not very obvious in the pigeon, in which 

 the feathers are more uniformly distributed than in 

 many birds. Apteria may, however, be seen in the 

 mid-ventral and mid-dorsal regions of the body, and 

 along the outer surfaces of the proximal segments of 

 the wings. 



D. External Apertures. 



a. Median apertures. 



i. The mouth is a wide slit-like opening in the long 

 pointed beak. 



ii. The cloacal aperture is a transverse slit, with 

 prominent lips, on the ventral surface of the 

 body, below the base of the tail. 



