File:The structure and life of birds (1895) (14563624410).jpg

Identifier: structurelifeofbhead (find matches)
Title: The structure and life of birds
Year: 1895 (1890s)
Authors: Headley, F. W. (Frederick Webb), 1856-1919
Subjects: Birds Evolution
Publisher: London and New York, Macmillan and co.
Contributing Library: Boston College Libraries
Digitizing Sponsor: Boston Library Consortium Member Libraries

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the vaso-motor nerves, which govern thearteries and so regulate the flow of blood. Andthrough it pass all of the twelve pairs of nerveswhich proceed from the brain, except two, the opticand olfactory; and these two are not, strictlyspeaking, nerves, but prolongations of the brain.The muscles that move the eyes, the muscles ofthe face, the tongue, the larynx, the lungs, the liver,and stomach work at the bidding of nerves that arisefrom the medulla oblonsrata. TJie Eye. In most essentials the birds eye is formed on thesame plan as our own. It is a camera at the back ofwhich is a nerve which expands into what is calledthe retina; the retina is sensitive to light, and theimage formed upon it is conveyed by the nerve to thebrain, where the impulse given to the nerve becomessensation—where, that is, sight actually takes place. Before describing the eye more particularly, I wish ^ For a description of the pineal body see Lubbocks Se?isespf Animals 122 THE STRUCTURE AND LIFE OP^ BIRDS chap.
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VI FORM AND FUNCTION 123 to distinguish sight from mere sensitiveness to light.Thus much even an earthworm possesses, for when atnight the hght of a lantern is thrown upon him hehurries into his hole. This is quite different fromseeing a definite image of things. With our e)esshut, we can tell whether we are in a bright light orin the dark, and the earthworm has no power beyondthis. An insects compound eye, again, is formed onquite a different principle from the eyes of vertebrateanimals. It has a number of tiny facets beneathwhich are sensitive cells, so that a mosaic picture isformed. There can be no doubt that eyes of thisdescription are very inferior to our own. Amongtheir great defects is this, that they have no powerof adjusting themselves to different distances. Toreturn to the vertebrate eye. It is a camera with abiconvex lens in front—i.e., a lens rounded outwards onboth sides. If a lens of this kind (a common magni-fying glass will do well) be taken and a candle beheld in

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