SEC. Ill EXTERIOR PARTS OF BIRDS 129 



ories, 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 

 poultry, being what a good cook finds it necessary to singe off after 

 plucking a fowl for the table. 5. Certain down-feathers are remark- 

 able for continuing to grow indefinitely, and with this unlimited 

 growth is associated a continual breaking down of the ends of 

 the barbs. Such plumulse, from being always dusted over with dry, 

 scurfy exfoliation, are called powder-dovm ,- they may be entitled to 

 rank as a fifth kind. I call them puhiplwmes. They occur in the 

 hawk, parrot, and gallinaceous tribes, and especially in the herons 

 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, 

 whitish feathers, matted over the hips and on the breast. 



Feather Oil Gland. — Birds do not perspire, and cutaneous glands, 

 corresponding to the sweat-glands and sebaceous follicles so common 

 in Mammalia, are little known among them. But their " oil-can " 

 is a kind of sebaceous follicle, which may be noticed here in connec- 

 tion with other tegumentary appendages. This is a two-lobed or 

 rather heart-shaped gland, saddled upon the " pope's-nose," at the 

 root of the tail, and hence sometimes called the uropygial gland 

 (Lat. uropygium, rump), or rump-gland. I have named it the elceo- 

 doehm ( Gr. eXaioS6)(o^, elaiodochos, containing oil ; Fig. 24, 9). 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 one or more pores, commonly upon a little 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 presence or absence of 

 this singular structure, and whether or not it is surmounted by a 

 particular circlet of feathers, distinguishes certain groups of birds, 

 and has come to be much used in classification. 



Pterylogpaphy. — Feathered Tracts and Unfeathered 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 

 covered with feathers, but it does not follow that feathers are every- 

 where implanted upon the skin. On the contrary, a uniform and 

 continuous pterylosis is the rarest of all kinds of feathering ; though 

 such occurs, almost or quite perfectly, among certain birds, as the 

 ostrich tribe, penguins, and toucans. If we compare a bird's skin 

 to a well-kept park, part woodland, part lawn ; then where feathers 

 grow is the woodland, where they do not grow is the lawn. The 

 former places are called tracts or pterylm (Gr. inepov, pteron, a plume 



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