294 ANIMAL PHYSIOLOGY. [LESSON 91. 



the combination of a vast number of detached portions of even a 

 large picture makes upon the optic lobe a perfect and undivided 

 whole, the impression of which is conveyed to the brain by the optic 

 nerve, not as a divided, but as a single picture. An Insect, there- 

 fore, has no business to know from any practical results that it pos- 

 sesses more than a single eye on each side of its head. 



1329. The necessity for such a vast number of distinct organs 

 of vision, is a consequence of their fixity of position ; it is essential 

 for their well being, to guard and protect themselves from their nu- 

 merous enemies, ever ready to destroy them, that they possess vision 

 in every conceivable direction, and the predaceous varieties require 

 a no less perfect development of the organ to enable them to discern 

 their nimble prey. The large extent of surface occupied by the 

 compound eyes, fully effects this desirable object ; above, below, be- 

 fore, behind and laterally, asleep or awake, are an infinite number 

 of vigilant guardians, never closed nor veiled by eyelids, but ever 

 on the alert and ready to give the alarm. 



1330. The number of distinct organs in the facetted eye has been 

 computed (by the aid of a micrometer), by those persons curious in 

 such matters, thus : the House-fly possesses 4,000 ; Libellula (Dragon- 

 fly), 12,554; Papilio, 17,355; Cossus ligniperda, 11,300; Mordella 

 (a small beetle), 25,088. 



1331. Hairs are frequently found connected with the compound 

 eyes ; they are placed in the depressions between the corneao, and 

 afford protection (like eyelashes) to the organs. 



1332. Bees have to enter the expanded cup of flowers, in search 

 of the nectar ; their eyes might suffer abrasion and become opaque 

 from the frequent contact with the petals, or they might be obscured 

 and rendered useless by aggregated pollen masses. But no such 

 contingencies can occur to them, in consequence of the protection 

 afforded by their eyelashes. 



1333. The telescopic, or simple eyes, are differently constructed. 

 Usually, these are of great size, as compared with any of the facets 

 of the compound eye. 



1334. Immediately behind the hemispherical, convex, trans- 

 parent cornea, is a well-shaped, double-convex, crystalline lens. 

 Like those already described, it possesses two curves, the deeper 

 one being behind. This latter surface fits accurately into a vitreous 

 humor, the posterior portion of which is rounded, and is received 

 into a bowl-shaped expansion of the optic nerve a true retina ; in 

 addition, there is a choroid coat, and a pigmentary layer, so that all 



