THE EXTERNAL STRUCTURE OF BIRDS. 139 
Feathers may be considered as very complex hairs of a conical 
form, which split up according toa definite pattern. Each is at 
first a little, soft, vascular process or papilla, curiously grooved. 
On one side is a central vertical groove, broadest at the base, 
and vanishing towards the apex of the papilla. Other less deep 
srooves, closely set, go out, nearly at right angles, from either 
side of this vertical groove. They extend almost all round the 
papilla, only vanishing towards the middle of the opposite side 
to that which bears the vertical groove. Grooves smaller still 
and much shorter are given. off again nearly at right angles 
from the grooves encircling the papilla, and sometimes others 
again from these. A horny secretion is deposited on the 
papilla, and is, of course, thickest where the grooves are deepest, 
and thinnest where there are no grooves at all—7. ¢., on the 
interspaces of the grooves. With the progress of growth, this 
whole horny investment splits up along the interspaces, where 
the deposit is thinnest. The part which was the main vertical 
groove is thickest of all, and becomes the shaft of the feather, 
the parts in the secondary grooves become the * barbs,” those 
on the still smaller ones the “ barbules,”” and those in the occa- 
sionally present yet smaller ones, the “ barbulets.” Sometimes 
a papilla will have a vertical groove on either side, and then the 
feather will have two shafts (one an aftershaft)—as in the 
Cassowary. The vane is the part of the case of the papilla 
which thus splits. The quill is that part of the case which does 
not split at all. At the upper end of the quill there must 
be a small perforation which marks just that spot where the 
feather ceases to open and flatten itself, and begins to remain 
curled round and continuous, as it all was at first. The space 
where it thus begins to remain curled round is the wmbilicus 
superior before mentioned. The papilla persists as the “ pulp” 
which ascends through the wmbilicus tufervor. 
Feathers are developed with great rapidity, sometimes attain- 
ing a length of two feet or more in a few days. They are also 
almost all renewed every year, and in many species twice a year. 
When we think of the serious effects of teething in mankind 
we cannot but be struck with the great vital energy of birds, 
and with the critical character of their process of moulting 
(ecdysis), which is, indeed, not unfrequently a fatal one. 
The annual moult commonly begins just after the close of 
the breeding-season, and it takes place in all Birds, from the 
Wren to the Ostrich. Such a process is obviously a necessary 
