AVliS. 157 



adapted to protect them from the rapid variations of temperature to which their move- 

 ments expose theui. The air-cavities which occupy the interior of their body, and 

 [usually] even sujiersede the mrjrow 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 

 the profundity of the globe to the edge of the crystalline, probably assists in displacing 

 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 

 inner angle, and which, by means of a remarkable muscular apparatus, can be drawn 

 over the front of the e)'e like a curtain. The cornea is very convex, but the crystalline 

 is flat, and the vitreous humour small. 



The ear of Birds has but a single small bone, formed of a branch adherent to the 

 tympanum, and of another terminating in a plate that rests upon the fenestra ovalis : 

 their cochlea is a cone slightly curved ; but their semicircular canals are large, and 

 lodged in a portion of the skull, where they are surrounded on all sides by air-cavities 

 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 

 covered with feathers [the ear-coverts'], the barbs of which are more fringed than those 

 of other feathers. 



The organ of smell, concealed within the base of the beak, has ordinarily three car- 

 tilaginous ossa tiirbinata, 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 

 openings of the nostrils determines the strength of the beak ; and the cartilages, 

 membranes, feathers, and other teguments which contract these apertures, exert an 

 influence on the perceptibility of odours, and on the sort of nourishment. 



The tongue has Uttle muscular substance, and is supported by a bone articulated on 

 the hyoid ; in most Birds this organ is not very delicate. [The Parrots probably enjoy 

 most perfectly the sense of taste.] 



The feathers, as well as the quills, which differ only in size, are composed of a stem, 

 hohow at its base, and of barbs, which are themselves furnished with smaller ones ; 

 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 vibrissas even, wdiich are disposed in some instances as eyelashes, and more 

 frequently impend the nostrils or arm the rictus of Birds, are merely barbless feathers, 

 which are developed and periodically renewed hke other feathers. In m.any groups, 

 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 

 iirimary shaft, so that two similar feathers grow from the same quill ; and in the 

 Cassowary, there is even a third shaft in addition. In the Poultry and some others, 



