HIGH SCHOOL ZOOLOGY. 



35 



43. The examination of the bulb itself (Fig. 13) discloses three 

 coats, the outermost of which is suVidivifled into an anterior trans- 

 parent part tlie comea, and a posterior hard fibrous and opacpie 

 part the sclerotic. Within this is the second coat, the choroid, 

 which chiefly serves to distriliute the blood within the bulb, 

 and to form a dark background for the retina, but anteriorly 

 forms a muscular sci-een — the iris, perforated Ijy the pupil, 

 throuo-h which the amount of light admitted to the sensitive 

 part of the eye may be regulated Vjy the iris. Suspended from 

 the junction of the choroid and iris by a sjjecial ciliary muscle, 

 is the lens a globular transparent body which changes tlie 



course of the ravs of liirht 



admitted to the eye, and 

 casts an inverted ima<;e on 

 the retina or nervous coat, 

 which lines the whole of the 

 choroid coat, and is separat- 

 ed from the lens by the fluid 

 and transparent vitreous 

 humor. The optic nerve 

 which terminates in the re- 

 tina, must therefore pierce 

 both the sclerotic and chor- 

 oid coats to do so, and in- 

 deed it perforates the re- 

 tina also, for its fibres form 

 the innermost of the several 

 layer's of which the retina 



Fi<' 14.— Diagrammatic Section of Retina of [^ composed. 

 joung Catfish. X400. ^ 



l.laverof optic nen-e fibres; 2, of ganglion 44^ Stiifly of 'the develop- 



f^lK ■ "s internal n^'>lecular, 4, internal ijranu- . , .. i j.i j. +k, 



UrorV'ai."lionicriaver(containin.,^ nuclei of Miil- ment of the eye shows that the 



ler's fibres) ; S, external molecular, C, external ,.g^j^a^ jg really an outgrowth of 



"•ranularlayer,containingnucleiofthero^Isand , , . ... ... ,. , . 



cones 8 but separated from them by the ex- the brain, which like the lirain 



ternal li'n.itin|X membrane, 7 ; 0, pigmentary ^^^ ^^^^ ^j ^^^^j^.j^ 



retinal einthuhum, "■"'^ " •' ' 



