AVES. 
157 
adapted to protect them from the rapid variations of temperature to which their move- 
ments expose them. The air- cavities w^hich occupy the interior of their body, and 
[usually] even supersede the marrow in their bones, increase their specific lightness. 
The sternal portion of the ribs is ossified, as well as the vertebral, to impart more force 
to the dilatation of the chest. To each rib is attached a small bone, which soon becomes 
soldered to it, and is directed obliquely backward towards the next rib, all concurring 
to give additional solidity to the thorax. 
The eye of Birds is so conformed as to enable them to distinguish objects both far 
and near with equal clearness ; a vascular and plaited membrane, which extends from 
i the profundity of the globe to the edge of the crystalline, probably assists in displacing 
I that lens. The anterior surface of the globe is also strengthened by a circle of bony 
pieces ; and, besides the two ordinary eyelids, there is always a third, situate at the 
I inner angle, and which, by means of a remarkable muscular apparatus, can be drawn 
j| over the front of the eye like a curtain. The cornea is very convex, but the crystalline 
I is flat, and the vitreous humour small. 
j| The ear of Birds has but a single small bone, formed of a branch adherent to the 
I tympanum, and of another terminating in a plate that rests upon the fenestra ovalis : 
li their cochlea is a cone slightly curved ; but their semicircular canals are large, and 
ij lodged in a portion of the skull, where they are surrounded on all sides by air-cavities 
i that communicate with the area. [Some] nocturnal Birds alone have a large 
external conch, which however does not project like that of quadrupeds, [though in the 
restricted genus Strix an overlapping cartilaginous flap is developed anteriorly, by 
which the auditory aperture is closed at will] . The orifice of the ear is generally 
I' covered with feathers [the ear- covert s'], the barbs of which are more fringed than those 
j of other feathers. 
i| The organ of smell, concealed within the base of the beak, has ordinarily three car- 
i tilaginous ossa turhinata, which vary in complication ; it is very sensible, although it 
! has no cavity excavated within the parietes of the cranium. The size of the bony 
I openings of the nostrils determines the strength of the beak; and the cartilages, 
j membranes, feathers, and other teguments which contract these apertures, exert an 
I influence on the perceptibility of odours, and on the sort of nourishment. 
' The tongue has little muscular substance, and is supported by a bone articulated on 
I I the hyoid ; in most Birds this organ is not very delicate. [The Parrots probably enjoy 
!! most perfectly the sense of taste.] 
j! The feathers, as well as the quills, which difleronly in size, are composed of a stem, 
I hollow at its base, and of barbs, which are themselves furnished with smaller ones ; 
I their tissue, lustre, strength, and general form, vary infinitely. [They may be con- 
veniently divided into clothing feathers, and those which are subservient to locomo- 
tion ; the vibrissae even, which are disposed in some instances as eyelashes, and more 
I frequently impend the nostrils or arm the rictus of Birds, are merely barbless feathers, 
' which are developed and periodically renewed like other feathers. In many groups, 
j the clothing feathers are furnished with a supplementary shaft, or accessory plume, 
which, in the quills or sustaining feathers, is at most represented by only a few downy 
filaments. This supplementary plume, in the Emeus, is developed equally with the 
primary shaft, so that two similar feathers grow from the same quiU : and in the 
Cassowary, there is even a third shaft in addition. In the Poultry and some others. 
