86 GENEBAL OBNITHOLOGY. 



them. Another feature is, that they are usually individually moved by subcutaneous muscles, 

 of which there may be several to one feather, passing to be attached to the sheath of the tube, 

 inside the skin, in which the stem is inserted. These muscles may be plainly seen under the 

 skin of a gO(.>se, and every one has observed their operation when a hen shakes herself after a 

 sand bath, or any bud erects its top-knot. 2. Down-feathers, pluimdiE, are cliaracterized by 

 a downy structure throughout. They more or less completely invest the body, but are almost 

 always hidden beneath the contour-feathers, Hke padding about the bases of the latter ; occa- 

 sionally they come to hght, as in the fleecy ruff about the neck of the condor, and then usually 

 replace contour-feathers ; they have an after-shaft, <n- none ; and sometimes no rhachis at all, 

 the barbs then being sessile in a tuft at tlie end of the quill. They often stand in a regular quin- 

 cunx {'.■'.) betAveen four contour-feathers. 3. Semiplumes, semiplumfp, may be said to unite 

 the characters of the last two, possessing the pennaceous stem of the former, and the plumula- 

 ccous vanes of the latter; they are with or without after-shaft. They stand among penna?, as 

 the plumulaj do, about the edges of patches of the foruicr, or in parcels by themselves, but arc 

 always covered by contour-feathers. 4. Filopliiiiies, jiloplumcc, or thread-feathers, have an 

 extremely slender, almost invisible stem, not well distinguished into barrel and shaft, and 

 usually no vane, unless a terminal tuft of barbs may be held for such. Long as they are, 

 they are usually hidden by the contour-feathers, close to which they stand as accessories, 

 one or more seeming to issue out of the very sacs in which the larger feathers are implanted. 

 These are the nearest approach to hairs that birds have ; they are very well shown on domestic 

 jioultry, being what a good cook finds it necessary to singe off after plucking a fowl for the 

 table. 5. Certain down-feathers are remarkable for continuing to grow indefinitely, and with 

 this unUmited growth is associated a continual breaking down of the ends of the barbs. Such 

 plumula', from being always dusted over Muth dry, scurfy exfoliation, are called poicder-down ; 

 they may be entitled to rank as a fifth kind, or pjulDiplwmes. They occur in the hawk, paiTot, 

 and gaUinaceous tribes, and especially in the hertjus and their allies. They are always present 

 in the latter, where they may be readily seen as at least two large patches of greasy or dusty, 

 wliitish feathers, matted over the hips and on the breast. The design is unknown. 



Featlier Oil Gland. — Birds do not perspire, and cutaneous glands, corresponding to tlie 

 sweat-glands and sebaceous foUicles so common in Mdmnialia, are little known amona- them. 

 But their "oU-can" is a Idnd of sebaceous foUiclc, which may be noticed here in counectiou 

 with other tegumentary appendages. This is a t.i\'o-lobed or rather heart-shaped gland, sad- 

 dled upon the " pope's nose," at the root of the tail, and hence sometimes called the Kropi/gial 

 (Lat. nropyrjimii, rump), or runip-glaud. If there be no single word to name it, it may be 

 called the elceodochon (Gr. eXaioSoxof, elniodochos, eontaining oil). It is composed of numerous 

 slender tubes or follicles which secrete the greasy fluid, the ducts of which, uniting successively 

 in larger tubes, finally open by cjue or more pores, commonly upon a Uttle nipple-like elevation. 

 Birds press out a drop of oil with the beak and dress the feathers with it, in the well-known 

 operation called " preening." The gland is large and always present in aquatic birds, which 

 have need of waterproof plumage ; smaller in land-birds, as a rule, and wanting in some. The 

 liresencc or absence of this singular structure, and whether or not it is surmoimted by a particu- 

 lar circlet of feathers, distinguishes certain groups of birds, and has come to be made much use 

 of in classification. 



Pterylography. — Feathered Tracts and Uiifeathered Spaces. — Excepting certain 

 birds having obviously naked spaces, as about the head or feet, all would be taken to be 

 fully feathered. So they are all covered tvith feathers, but it does not follow that feathers are 

 everywhere implanted upon the skin. On the contrary, a, uniform and continuous pteryhsis 

 is the rarest of all kinds of feathering ; though such occurs, almost or quite perfectly, among 



