274 THE ANATOMY OP VERTEBEATED ANIMALS. 



The exoskeleton of Birds consists almost entirely of epi- 

 dermic structures in tlie form of homy sheaths, scales, 

 plates, or feathers. No bird possesses dermal ossifications, 

 unless the spurs which are developed upon the legs and 

 wings of some species may be regarded as such. 



The feathers are of various kinds. Those which exhibit 

 the most complicated structure are called pen^ice, or contour 

 feathers, because they lie on the sui-face and determine the 

 contour of the body. In every penna the following parts 

 are to be distinguished : — 1. A main stem (scapus) forming 

 the axis of the feather, and divided into a proximal hollow 

 cylinder, partly embedded in a sac of the derm, called the 

 calamus, or quill ; and a distal vexillum, or vane, consisting 

 of a four-sided solid shaft, the rachis, which extends to the 

 extremity of the feather, and bears a number of lateral pro- 

 cesses, the barbs. The calamus has an inferior aj^erture 

 {umbilicus inferior), into which the vascular pulp penetrates; 

 and a superior aperture {umbilicus superior), situated on the 

 under surface of the feather at the junction of the calamus 

 with the scapus. The barbs are narrow plates, tapering to 

 points at their free ends, and attached by their bases on 

 each side of the rachis. The edges of these barbs are 

 directed upwards and downwards, when the vexillum of 

 the feather is horizontal. The interstices between the 

 barbs are filled up by the barbules ; pointed processes, which 

 stand in the same relation to the barbs, as the barbs do 

 to the rachis. The barbules themselves may be laterally 

 serrated and terminated by little hooks, which interlock with 

 the hooks of the opposed barbules. In very many birds 

 each quill bears two vexilla ; the second, called the aftershaft 

 {hyporachis), being attached on the under side of the first 



2F. — Galling (with Columbx) = Akclor(miorphce, Peristeromorpha. 



PteroclomorplicB, Tumicimorpha. 



V. — Cdrsores = Satitw. 



VI. — Grallje = CharadriomorphcB, GeranomorphcE, 



AmphimorphcBf PelargomarplttE. 



VII. — Palmipedes .... — Cecomorph<E, Spheniscomorphcs, 



ChenomorphcB, Dysporomorphcd. 



