ANIMAL PHYSIOLOGY 



77 



of a transparent light-refracting lens lying outside of the 

 retina and exposed to the light. These are the constant 

 essential parts of an image-forming and image-perceiving 

 eye. In most eyes there are other accessory parts which 

 may make the whole eye an organ of ex- 

 cessively complicated structure and of re- 

 markably perfect seeing capacity. Our 

 own eyes (fig. 33) are organs of extreme 

 structural complexity and of high devel- 

 opment, although some of the other ver- 

 tebrates have undoubtedly a keener and 

 more highly perfected sight. 



The crustaceans and insects have 

 eyes of a peculiar character called com- 

 pound eyes (figs. 34 and 35). In ad- 

 dition most insects have smaller simple 

 eyes. Each of the compound eyes is 

 composed of many (from a few, as in 

 certain ants, to as many as twenty- 

 five thousand, as in certain beetles) 

 eye elements, each eye element seeing 

 largely independently of the others and 

 seeing only a very small part of any ob- 

 ject in front of the whole eye. All the 

 small parts of the external object seen 

 by the many distinct eye elements 

 combine so as to form an image in mo- 

 saic, that is, made up of separate small 

 parts of the external object. If the head 

 of a dragon-fly be exammed it will be 

 seen that two-thirds or more of the whole 

 head is made up of the two large compound eyes, and with a 

 lens it may be seen that the outer surface of each of these eyes 

 is composed of many small spaces or facets, which are the out- 

 er lenses of the many eye elements composing the whole eye. 



F1G.3S. Section through 

 a few facets and eye 

 elenients(omniatidia) 

 of the compound eye 

 of a moth; /, corneal 

 facets; c. c, crystalline 

 cones; p, pigment; r, 

 retinal parts; o. «., 

 optic nerve. (Greatly 

 magnified; after Ex- 

 ner.) 



