COLUMBA. 361 



tongue. Immediately behind the tongue is a median slit, 

 the glottis. Dorsally lie the two internal ?iares^ and behind 

 them is a single Eustachian aperture which soon diverges 

 into the two Eustachian canals to the ears. At the hind- 

 end below the tail is a single median cloaca I aperture. The 

 fore-limbs are formed into wings and the hind-limbs form 

 the legs. There are four toes terminating in claws. 



The whole body, wdth the exception of the beak and the 

 lower part of the legs, is completely enveloped in a coat of 



integumentary. /'^^^f^; .^ ^^^^^^^' structurally as well as 

 popularly, is an organ sm generis. Nothmg 

 quite like it is found in any group outside the birds. 

 A feather arises from the epidermis and remains attached 

 to the skin by' its base. If the feathers be plucked or 

 pulled out of their epidermic pits, it is seen that they are 

 attached only on certain areas of the skin called pterylcE^ the 

 portions of bare skin between them being called apteria. 



The skin itself is dry and powdery, and there is an 

 absence of the numerous skin-glands found in the frog, with 

 the exception of the IsiXge preen-gland dX the base of the tail. 

 This involves the " preening " of the feathers by the bird, in 

 which process the greasy secretion from the gland is spread 

 by the bird's beak over each feather. 



The largest feathers are found in the wing and tail and 

 form the quill- or flight-feathers. The central axis of the 

 feather is hollow in its lower part, called the guill. Open- 

 ing into the hollow cavity is a small aperture at the base, 

 called the inferior umbilicus ; and at the distal end of the 

 quill region is a smaller superior umbilicus.'''^ Above the 

 quill the axis is extended as the solid shafts bearing on either 

 side the vane. The vane or flattened part is formed of a 

 great number of parallel barbs attached basally to the shaft 

 and laterally to each other by small interlocking processes 

 or barbules. 



The quills on the wing are called remiges and those of 

 tail are rectrices. A remex usually is more tapering, and has 

 the vane very unequal in size in comparison with a rectrix. 



* This peculiar structure is explained by the development of the feather from a 

 single tube, of which the part above the superior umbilicus splits longitudinally and 

 spreads out to form the vane and shaft, leaving the quill to open to the exterior by 

 the superior umbilicus. 



