PLUMAGE : TRACTS AND SPACES. 



rot, and gallinaceous tribes, but especially in the heron family, where they 

 are alwaj^s present, and readily seen as two large patches of greasy or dusty, 

 whitish, matted feathers over the hips and in front of the Ijreast. Their use 

 is not known. 



§ 7. Feather Oil-gland. With comparatively few and irregular ex- 

 ceptions, birds have a singular apparatus for secreting oil with which to lu- 

 bricate and polish their feathers. It is a two-lobed, or rather heart-shaped, 

 gland, saddled upon the root of the tail; consisting essentially of numerous 

 slender secreting tubes or follicles, the ducts of which succcssivel}^ unite 

 in larger tubes, and finally joerforate the skin at one or more little nipple- 

 like emiuences. Birds press out a drop of oil with their )jcak, and then 

 dress the feathers with it. The gland is largest in water-birds, which have 

 most need of an impervious coating of feathers, and alwaj's present among 

 them ; very large in the fish-hawk; smaller in other land-))irds, and want- 

 ing (it is said), among tlie ostriches, bustards, parrots and some others. (In 

 pi. I, fig. 4, the line 6 points to the oil-gland.) 



§ 8. Development of Feathers. In a manner analogous to that of 

 hair, a feather grows in a little pit or pouch formed by inversion of the der- 

 mal layer, and is formed in a closed oval follicle consisting of an inner and 

 outer coat separated by a layer of fine granular substance. The outer layer, 

 or "outer follicle " is composed of several thin strata of nucleated epithelial 

 cells ; the inner is thicker, spongy and filled with gelatinous fluid ; a little 

 artery and vein furnish the blood-circulation. The inner is the true matrix of 

 the feather, evolving from the blood-supply the gelatinous matter, and resolv- 

 ing this into cell nuclei ; the granular layer is the formative material. The 

 outer grows a little beyond the cutaneous sac that holds it, and opens at 

 the end ; from this orifice the future feather protrudes as a little, fine-rayed 

 pencil point. During subsequent growth the follicular laj^ers undergo little 

 further change ; it is the granular that becomes the feather. 



§ 9. All a bird's feathers, of whatever kind and structure, taken together, 

 constitute its ptilosis or 



Plumage. 



(a.) Feathered Tracts and Unfeathered Spaces. With the exception 

 of certain birds that have obviously naked spaces, as aljout the head, etc., all 

 would be taken to be fully feathered. So they are full}^ covered with feath- 

 ers ; but it does not follow from this, that feathers are implanted everywhere 

 upon the skin. On the contrar}'-, this is the rarest of all kinds of feather- 

 ing, though it occurs, almost or quite perfectly, among the penguins and 

 toucans. Let us compare a bird's skin to a well-kept park, part woodland, 

 part lawn ; then where the feathers grow is the woodland ; where they do 

 not grow, the lawn ; the former places are called tracts {pterijloi) ; the latter 

 spaces {cipteria') ; they mutually distinguish each other into certain definite 

 areas. Not only are the tracts and spaces thus definite, but their size, form 

 and arrangement mark whole families or orders of liirds, and so are impor- 



