The hind-limbs and tail.—The hind-limbs, or legs, of birds are the sole support of the body except during flight, and a relatively great weight is thus thrown upon them. To provide for this, the skeleton of the hip region is exceptionally strong. Not only are the bones of the spine here welded together into one solid mass, but the [Pg 276] hip-bones extend forward much further than is the case in quadrupeds, and are firmly attached to the fused vertebrae. Each hip-bone possesses a socket (actb., Fig. 182) into which fits the head of the bone of the upper leg. Another fusion of bones is to be noticed at the end of the tail, resulting in the ploughshare-bone (Fig. 182, pyg. st.) which supports the large steering feathers of the tail. The quills of these feathers are covered at their bases by the tail-coverts. Just above the end of the tail of a plucked pigeon is to be seen a small conical body, the oil-gland, from which the bird obtains a fluid which it applies to the feathers during the preening process. The feathers.—Feathers do not grow upon all parts of a bird’s body, but are restricted to certain definite areas or “feather tracts” which, however, differ in arrangement in different species. Fig. 183.—Pigeon. A, portion of a wing feather; cal, quill; rch, shaft; B, filoplume; C, nestling-down. One of the large feathers of the wing of the pigeon is illustrated in Fig. 183, A. It consists of a hollow, horny quill (cal.), prolonged into a solid shaft (rch.) which supports the web or vane. The outer face of the feather is slightly convex and smooth; while the inner surface is somewhat concave and rough, and carries a little tuft of down at the junction of quill and shaft. The growing feather is nourished by a conical projection of the deeper part of the skin, which fits into a small hole (inf. umb.) at the base of the quill. It is easily shown that the vane of the feather is composed, on each side, of a large number of parallel laths which spring from the [Pg 277] shaft. These laths are called barbs. Considerable resistance is felt when an attempt is made to pull the barbs apart, but the manner of their connection cannot be clearly seen without the aid of the microscope. A fairly low magnifying power, however, shows (Fig. 184) that each barb bears extremely delicate threads—barbules—on each side, arranged on the barb much as the barbs are arranged on the shaft. The barbules on one side of each barb (the side furthest from the quill) are seen to carry tiny hooks; while the barbules of the other side of the barb are furnished with flanges. The hooked barbules of one barb cross the flanged barbules of the next and interlock with them; so that relatively great force is required to pull the barbs apart and destroy the continuity of the web. The quill-feathers of the tail, and the coverts, have webs of similar structure.