432 DISSECTION OF THE PIGEON. 



there is a second aperture, the superior um- 

 bilicus. 



ii. The rachis, or shaft, is the distal solid portion 



of the stem. It is a tapering, flexible, elastic 



rod, square in transverse section, and grooved 



along its ventral surface. 



b. The vexillum, or vane, is the flattened portion of 



the feather, attached along the sides of the 



rachis. It is made up of barbs and barbules. 



i. The barbs are a series of narrow elastic laminae, 

 attached by their bases along the two sides 

 of the rachis, perpendicular to the plane of 

 the vane. 



ii. The barbules are much smaller processes, which 

 form fringes along the sides of the barbs : 

 they bear hooklets, which hold together the 

 adjacent barbs, and so give the feather that 

 resistance to the air on which the bird's power 

 of flight depends. 



2. Varieties of feathers. 



a. The quill feathers are the large feathers of the 

 wings and tail : they are of two kinds, 

 i. The remiges, or wing-quills, have the inner or 

 posterior half of the vane much broader than 

 the outer or anterior half. They are divided 

 into groups in accordance with the bones to 

 which they are attached (fig. 75, p. 440). 



The primary quills are those attached to 

 the bones of the manus. They are eleven in 

 number, of which six are attached to the 

 middle metacarpal, and are hence called meta- 

 carpal quills : one, ad-digital, is attached 

 to the phalanx of the postaxial digit ; two, 

 mid-digital, to the proximal phalanx of the 

 middle digit ; and two, pre-digital, one of 

 which is small, are attached to the distal 

 phalanx of the middle digit. 



