File:Outlines of comparative physiology touching the structure and development of the races of animals, living and extinct - for the use of schools and colleges (1870) (14596036538).jpg

Identifier: outlinesofcompar00agas (find matches)
Title: Outlines of comparative physiology touching the structure and development of the races of animals, living and extinct : for the use of schools and colleges
Year: 1870 (1870s)
Authors: Agassiz, Louis, 1807-1873 Gould, Augustus A. (Augustus Addison), 1805-1866 Wright, Thomas, 1809-1884
Subjects: Physiology, Comparative Zoology
Publisher: London : Bell & Daldy
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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cted into groups, like those of spiders ; each eye inclosinga crystalline lens and a vitreous body, surrounded by a retinaand choroid. Such eyes consequently form a natural transi-tion to the compound eyes of insects and Crustacea, to whichwe now give our attention. § 142. Compound eyes have the same general form as simpleeyes ; they are placed either on the sides of the head, as ininsects, or supported on pedicles, as in crabs. If we examinean eye of this kind by a magnifying lens, we find its surface com-posed of an infinite number ofangular, usually six-sided facettes(fig. 44). If these facettes are re-moved, we find beneath, a corre-sponding number of cones (c),side bv side, five or six times as V long as they are broad, and ar-ranged like rays around the op-tic nerve, from which each onereceives a little filament, so as topresent, according to Midler,the following disposition. Thecones are perfectly transparent,hut separated from each other by walls of pigment, in such. Fig. 44.
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TO SPECIAL SENSES. a manner, that only those rays -which are parallel to the axescan reach the retina (A) ; all those which enter obliquely arelost; so that of all the rays which proceed from the points aand b, only the central ones in each pencil act upon theoptic nerve, d: the others strike against the walls of thecones. To compensate for the disadvantage of such an ar-rangement, and for the want of motion, the number of fa-cettes is greatly multiplied, so that no less than 25,000 havebeen counted in a single eye. The image on the retina, in thiscase, may be compared to a mosaic, composed of a great num-ber of small images, each of them representing a portion ofthe figure. The entire picture is, of course, more perfect, inproportion as the pieces are smaller and more numerous. § 143. Compound eyes are destitute of the optical appa-ratus necessary to concentrate the rays of light, and cannotadapt themselves to the distance of objects ; they see at a cer-tain distance, but cannot loo

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