. A manual of zoology. Zoology. IV. VERTEBRATA: AVE.'^. (ill etc.) well developed. The beginnings of an external ear are seen in the deeper position of the drum membrane. Since the power of flight necessitates vision at great distances, most birds have exceed- ingly sharp sight, and the eye itself (fig. 042) is in general con-. Fig. 642.—Eye of owl. (From Wiedersheim.) Ch, choroid; Cil/, ciliary muscle; Co, cornea; Cf, vitreous body; Ir, iris ; jD, lens; Op, optic nerve; OS, sheath of nerve; P, pecten; Rt, retina; Sc, sclera; VK, anterior chamber; +, sclerotic bones. structed for distance. Pec
. A manual of zoology. Zoology. IV. VERTEBRATA: AVE.'^. (ill etc.) well developed. The beginnings of an external ear are seen in the deeper position of the drum membrane. Since the power of flight necessitates vision at great distances, most birds have exceed- ingly sharp sight, and the eye itself (fig. 042) is in general con-. Fig. 642.—Eye of owl. (From Wiedersheim.) Ch, choroid; Cil/, ciliary muscle; Co, cornea; Cf, vitreous body; Ir, iris ; jD, lens; Op, optic nerve; OS, sheath of nerve; P, pecten; Rt, retina; Sc, sclera; VK, anterior chamber; +, sclerotic bones. structed for distance. Peculiarities of the bird's eye, already weakly developed in the reptiles, are the pecten (P), a comb- shaped growth of the choroid into the vitreous body, and the scleral ring, a circle of bones developed in the sclera and support- ing the outer part of the eye. Among birds there is spirited rivalry for the females, especially among polygamous species. At the time of mating the males seek to win the favor of the females either through striking motions (dances), by singing, or by beauty of plumage. All of these peculiarities are confined to the male and frequently lead to a marked sexual dimorphism. The dis- tinction in plumage is commonly strengthened at this time, the male receiving the brilliant wedding dress. Thus we speak of the spring molt, although there is only a color change and only exceptionally a renewal of the feathers. The return to every-day clothes only occurs with a molt, and this comes at the close of the reproductive season. The reason for the dull plumage of the female is due to the fact that she usually sets on the nest, at which time inconspicuous colors protect her from destruction by enemies. In only a few instances is the heat neces- sary for incubation produced by other causes, such as the heat of the sun upon the sand in which the eggs are buried, or the increase of temperature caused by fermentation in decaying vegetation (Megapodes). The rule is. Ple
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectzoology, bookyear1902
