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- Fic.35. Section through 

 ject in front of the whole eye. All the a few facets and eye 



QTnall nart* nf the paternal ohiert seen elements(ommatidia) 

 small parts ol of the compound eve 



by the many distinct eye elements 

 combine so as to form an image in mo- 

 saic, that is, made up of separate small 



of a moth; /, cornea 1 

 facets; c. c, crystalline 

 cones; p, pigment; r, 

 retinal parts; o. n.. 



parts of the external object. If the head optic nerve. (Greatly 

 of a dragon-fly be examined 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. 



magnified; after Ex- 



ner. ) 



