| Fig. 2.—Pterylosis of the plover. |
In its simplest form it runs from the throat backwards in the form of a median band as far as the base of the neck where it divides, sending a branch to each side of the breast. This branch commonly again divides into a short, broad outer branch which lodges the “flank” feathers, and a long, narrow, inner branch which runs backwards to join its fellow of the opposite side in front of the cloacal aperture. This branch lodges the abdominal feathers. The median space which divides the inner branches of the tract may be continued forwards as far as the middle of the neck, or even up to the throat, e.g. plover. Only in a few cases is the neck continuously covered by the fusion of the dorsal and ventral tracts, e.g. flamingo, Anseres, Ciconidae, Pygopodes.
For convenience sake the cervical portions of the spinal and ventral tracts are generally regarded as separate tracts, the pt. colli dorsalis and pt. colli ventralis respectively.
(4) The humeral tract (pt. humeralis), which gives rise to the “scapular” feathers.
(5) The femoral tract (pt. femoralis), which forms an oblique band across the thigh.
(6) The crural tract (pt. cruralis), which clothes the rest of the leg.
(7) The tail tract (pt. caudalis), including the tail feathers and their coverts; and
(8) The wing tract (pt. alaris). The wing tract presents many peculiar features. Each segment—arm, forearm and hand—bears feathers essential to flight, and these are divided into remiges, or “quill” feathers, and tectrices, or “coverts.”
The remiges of the arm, more commonly described as “tertiaries,” are, technically, collectively known as the parapteron and hypopteron, and are composed respectively of long, quill-like feathers forming a double series, the former arranged along the upper, and the latter along the lower aspect of the humerus. They serve to fill up the gap which, in long-winged birds, would otherwise occur during flight between the quill-feathers of the forearm and the body, a gap which would make flight impossible. In short-winged birds these two series are extremely reduced.
The remiges range in number from 16, as in humming-birds, to 48 as in the albatross, according, in short, to the length of the wing. But these numerical differences depend, in flying birds, rather upon the length of the forearm, since the quills of the hand never exceed 12 and never fall below 10, though the tenth may be reduced to a mere vestige.