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. 3 i), 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 "quilP^ or calamus (3). 

 The vane is made up of a number of flattened plates know as barbs or 

 rami (fig. 3^ 1) set obliquely on the shaft and held together by a 

 very complex arrangement of interlocking processes called barbules or 

 radii (fig. 3^ 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 im|)ervious 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. 84). 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-piuraes, and Powder-down feathers. 



Contour-feat Iters 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 continuous vanes ; those of the wings and 



