. The structure and life of birds . entre 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 arc not, strictlyspeaking, nerves, but prolongations of 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 oblongata. The Eye. In most essentials the birds eye is formed on thesame plan as our own. It is a camera
. The structure and life of birds . entre 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 arc not, strictlyspeaking, nerves, but prolongations of 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 oblongata. The 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 1 For a description of the pineal body see Lubbocks Se?isespf Animah THE STRUCTURE AND LIFE OF BIRDS vi FORM AND FUNCTION ^3 to distinguish sight from mere sensitiveness to light:Thus much even an earthworm possesses, for when atnight the light of a lantern is thrown upon him hehurries into his hole. This is quite different fromseeing a definite image of things. With our eyesshut, we can tell whether we arc 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 arc sensitive cells, so that a mosaic picture isformed. There can be no doubt that eyes of thisdescription arc 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—, a lens rounded outwards onboth sides. If a lens of this kind (a common magni-fying glass will do we
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