STRUCTURE OF BIRDS. 77 



of minute silky filaments. This arrangement is especially re- 

 markable in the Gallinaceous birds. Frequently the filament 

 becomes spirally twisted, in which case the barbs seem to have 

 a circular arrangement, although they are still biserial. 



With respect to relative magnitude, the following is an ac- 

 count of the ordinary distribution of the feathers. From the 

 head, backw^ards to the tail, they increase in length and size ; 

 those on the face, or around the base of the bill, being smallest, 

 the tail-coverts lonofest. The w4no[-feathers are much shorter 

 than those of the body, and also increase backwards. Those 

 of the upper or dorsal half of the body are almost ahvays 

 shorter than those of the lower or abdominal ; and the dispro- 

 portion seems to have reference to the degree of obliquity of 

 the body in its ordinary posture; for in those birds which 

 have a nearly vertical position, such as Penguins, Auks, and 

 Guillemots, the feathers of the lower surface are scarcely 

 longer than those of the upper. The feathers of the upper 

 parts are also more compact than those of the lower. 



Besides the feathers properly so called, there enters into the 

 composition of the plumage or general envelope, another modi- 

 fication of the same general nature. On removing the wdiole 

 of the feathers whose tips appear externally, in certain orders, 

 and especially in the aquatic birds, we find the skin still co- 

 vered with a more or less dense envelope of a very soft, fila- 

 mentous, highly flexible, and very elastic substance, the down. 

 It also consists of individual parts, plumules or down-feathers, 

 and is analogous to the soft hair or fur of quadrupeds, while 

 the feathers themselves represent their long hair or pile. 



The iilumule or down-feather consists of two parts : — a small 

 tube, very narrow, and soft in texture, not well defined in its 

 lower or proximal part, and having its walls composed rather 

 of soft scales than of one continuous piece ; and a pencil of 

 filaments issuing from the base of this tube internally, without 

 any connecting shaft. These filaments vary in length and 

 number according to the species. In all cases they are ex- 

 tremely slender, pliant, sinuous, and more or less spirally 

 twisted. They consist of an extremely delicate shaft, along 

 the sides of which there come off, in general, two sets of short 



