PLUMAGE : TRACTS AND SPACES. 5 
rot, and gallinaceous tribes, but especially in the heron family, where they 
are always present, and readily seen as two large patches of greasy or dusty, 
whitish, matted feathers over the hips and in front of the breast. Their use 
is not known. 
§ 7. Frearner Om-cranp. 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 successively unite 
in larger tubes, and finally perforate the’skin at one or more little nipple- 
like eminences. Birds press out a drop of oil with their beak, 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 always present among 
them ; very large in the fish-hawk; smaller in other land-birds, and want- 
ing (it is said), among the ostriches, bustards, parrots and some others. _ (In 
pl. 1, fig. 4, the line 6 points to the oil-gland.) 
§ 8. DevrLorpment or Frearners. 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 resoly- 
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 layers undergo little 
further change; it is the granular that becomes the feather. 
§ 9. Alla 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 about the head, ete., all 
would be taken to be fully feathered. So they are fully covered with feath- 
ers; but it does not follow from this, that feathers are implanted everywhere 
upon the skin. On the contrary, 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 éracts (pteryl@) ; the latter 
spaces (apteria) ; 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 birds, and so are impor- 
