137 
APPENDIX ON THE STRUCTURE OF BIRDS. 

[The specimens illustrative of this subject are set out chiefly in the 
recess No. 4 of the Central Hall. | 
The Class Birds—Aves—may be briefly characterised as warm- 
blooded, egg-laying (oviparous), vertebrate animals, covered with 
feathers and having the fore-limbs modified into wings. Of these 
characters, the covering of feathers is alone sufficient to distinguish 
Birds from all other animals. 
Feathers [Figs. 1.-3'.|—The feathers of Birds correspond to 
the scales of Reptiles. A typical feather consists of a long tapering 
shaft or stem (rhachis) (fig. 31), bearing on each side for the 
greater part of its length a broad elastic web or vane (2). The part of 
the shaft to which the vanes are attached is four-sided, solid, grooved 
along its under surface, and very pliant. Below the vane, the stem is 
hollow and transparent, and is known as the “quill” or calamus (3). 
The vane is made up of a number of flattened plates know as barbs or 
rami (fig. 834 1) set obliquely on the shaft and heid together by a 
very complex arrangement of interlocking processes called barbules or 
radii (fig. 34 2). Where these barbules are perfectly developed 
and unite the barbs, the vane forms a continuous web, able to 
withstand the resistance of the air encountered during flight, and 
more or less impervious to water. In flightless birds the barbules 
are degenerate, and the barbs of the feathers being no longer held 
together are said to be discontinuous, as in the Ostrich-tribe, or in the 
tail-feathers of the Lyre-bird. 
In many feathers a small shaft bearing a discontinuous vane is found 
attached to the base of the under surface of the main shaft where it 
passes into the quill. This is called the aftershaft (hyporhachis) 
(fig. 3 4). In the Cassowaries and Emus among the Ostrich-tribe, 
and in the feathers of some nestling birds, this aftershaft equals the 
main shaft in size. 
Five kinds of feathers may be distinguished, viz. :—Contour-feathers, 
Semiplumes, Down-feathers, Filo-plumes, and Powder-down feathers. 
Contour-feathers are those which, as their name implies, determine 
the outline of the body, that is to say, they are all that meet the eye in 
the living bird. Those covering the head and body are more or less 
firm in structure and have continvous vanes; those of the wings and 
