5o6 BIRDS. 



dorsal surface at the root of the tail, the cloacal aperture, 

 are external features easily demonstrated. 



Feathers. — The feathers most important in flight are the 

 remiges of the wing, divided into primaries borne by the 

 fingers, and secondaries by the ulna. The feathers of the tail 

 help to guide the flight, and are called rectrices. A distinct 

 tuft of feathers borne by the thumb is called the bastard 

 wing. Covering the bases of the large feathers are the 

 coverts, — wing-coverts and tail-coverts, — while other small 

 feathers give shape to the whole body. In the pigeon there 

 are no true down feathers or plumules, but among the 

 ordinary contour feathers or pennte, there are little hair- 

 like feathers called filoplumes, which bear only a few ter- 

 minal barbs. 



Any one of the large feathers consists of an axis or scapus 

 divided into a lower hollow portion — the calamus or quill, 

 and an upper solid portion — the rachis, which forms the 

 centre of the vane. This vane consists of parallel rows of 

 lateral barbs, linked to one another by barbules, which may 

 be joined to one another by microscopic booklets. The 

 quill is fixed in a pit or follicle of the skin with which 

 muscle fibres are connected. At the base of the quill there 

 is a little hole — the inferior umbilicus — through which a 

 nutritive papilla of dermis is continued into the growing 

 feather. At the base of the vane there is a little chink — the 

 superior umbilicus — but this has no importance. Close to 

 this region, however, in many birds a tiift or branch arises 

 which is called the aftershaft. In the Emu and Cassowary, 

 the aftershaft is so long that each feather seems double. 



A feather grows from a papilla of skin, but the whole of 

 the feather is really formed from the cornification of one 

 layer of the epidermis. The papillae rarely occur diffusely 

 on th.e skin, but are usually disposed along definite lines or 

 feather-tracts, where the skin has been wrinkled. Each 

 papilla consists externally of epidermis and internally of 

 dermis, and becomes surrounded by a depression or moat, 

 which deepens to form the feather-follicle or the sac in 

 which the base of the quill is sunk. But the epidermis has 

 two layers — (a) an outer stratum corneum, which in the 

 developing feather forms merely a protective external 

 sheath, and {b) an inner stratum Malpighii, which be- 



