| OCR Text |
Show 220 AVES. able muscular apparatus, can be drawn over the eye like a curtain. The cornea is very convex, but the crystalline is flat, and the vitreous humour small. The ear has but a single small bone, formed of one branch that adheres to the tympanum, and of another terminating in a plate that rests upon the fenestra ov~lis_; the cochlea is a slightly arcuated cone; but the semi-Circular canals are large, and lodged in a part of the cranium where they are completely surrounded by air cavities, which communicate with the area. Nocturnal Birds alone have a long external conch which however, does not project like that of Quadru· peds. ' The ex' ternal meatus is generally covered W.i th feat he rs, whose barbs are more fringed than the others. The organ of smell, concealed in the base of the beak, usu· a1ly has but three cartilaginous ossa turbinata, which vary in complication; although there are no sinuses within the parietes of the cranium, it is extremely sensible. The breadth of the osseous openings of the nostrils determines the strength of the beak; and the cartilages, membranes, feathers and other teguments which narrow down those apertures, influence the power of smell, and the nature of the food. . There is but little muscular substance in the tongue, whtch is supported by a bone articulated with the hyoid; in most Birds this organ is not -very delicate. . . . The feathers as well as the quills, which only differ 1n size, are composed o'f a stem, hollow at base, and of barbs, wh l' C h are themselves furnished with smaller ones; their tissue, lustre, strength, and general form vary infinitely. The touch must be feeble in all such parts as are covered with them, and~ the beak is almost always corneous, and has but little sensl· bility, and the toes are in vested with scales above, and a callo~s skin underneath, that sense can have but little acti' vi' ty I' ll thiS class of animals. BI. rd s moult twi· ce a year. 1n certa·m speC·ie s, the winter plumage differs in its colours from· that of summer; in. the 1 · mfe· greater number, the female differs from the rna em an f rior vividness of tints, and when this is the case, the youngo AVES. 221 both sexes resemble the former. When the adult male and female are of the same colour, the young ones have a livery peculiar to them. The brain of Birds has the same general characters as that of other Oviparous Vertebrata, but is distinguished by its very great proportionate size, which often surpasses even that of this organ in the Mammalia. This volume principally depends upon tu~ercles, analo!Sous to the corpora striata, and not upon the hemispheres, whiCh are narrow and without circumvolutions. The cerebellum is tolerably large, and almost without lateral lobes, being chiefly constituted by the vermiform process . . The ~ings of the trachea are entire ; there is a glottis at its h1f~rcat~on most commonly furnished with peculiar muscles, whwh. IS call~d t~e inje1·ior larynx; this is the spot where the votce of birds IS produced; the immense volume of air contained i~ the ~ir sacs contributes to its strength, and the trachea, by Its vanous forms and motions, to its modifications. The superior larynx, which is extremely simple, has but little to do with it. :h~ face, or upper mandible of Birds, consisting chiefly of their mtermaxillaries, is lengthened out behind into two arches the internal of which is composed of the pterygoid and pal a tin~ ho~es, and the external of the maxillaries and j ugals, both of whtch rest on a movable tympanic bone, commonly caUed the square hone, analogous to that of the drnm of the ear · above . th I' s same face is articulated with the cranium, or uni't ed to i't by elastic laminre, a kind of union which always allows the parts some degree of motion. The horny substance which invests the two mandibles, performs the office of teeth, and is sometimes so jagged as to re .. semble them; its form, as well as that of the mandibles which supp or t I· t, vari·e s extremely, and according to the kind of food used by each species. )' The digestion of Birds is in proportion to the activity of their Ife, and the force of their respiration. The stomach is composed of three parts : the crop, which is an enlargement of |
