XIII 



PHYLUM CHORDATA 



107 



not the same as that of the sclerotic ; the former is almost flat in 

 Fishes, but bulges outwards in terrestrial Vertebrates. 



Lining the sclerotic is the second coat of the eye the choroid (ch.) 

 formed of connective-tissue abundantly supplied with blood- 

 vessels. At the junction of sclerotic and cornea, it becomes con- 

 tinuous with a circular membrane (/), placed behind but at some 

 distance from the cornea, and called the iris. This latter is strongly 

 pigmented, the colour of the pigment varying greatly in different 

 species, and giving, as seen through the transparent cornea, the 

 characteristic colour of the eye. The iris is perforated in the centre 



o.w 



Fio. 794. Diagram of the retina, the supporting structures to the left, the nervous and epithelial 

 elements to the right, a d, fibrous supporting structures ; gr. i/r'. granular layers ; n.c., n.c'. 

 n.c"., n.c"'. nerve-cells ; nu. nuclear layer of rods and cones ; o. n. fibres of optic nerve ; 

 r. rods and cones. (From Wiedersheim's Vertebrata, after Stohr.) 



by a circular or slit-like aperture, the pupil, which, in the entire 

 eye, appears like a black spot in the middle of the coloured portion. 

 Except in Fishes, the pupil can be enlarged by the action of a set 

 of radiating unstriped muscle-fibres contained in the iris, and con- 

 tracted by a set of circular fibres ; and the anterior or outer portion 

 of the choroid, where it joins the iris, is thrown into radiating folds, 

 the ciliary processes (C. P.), containing unstriped muscular fibres, 

 the ciliary muscle. 



Lining the choroid and forming the innermost coat of the eye is 

 a delicate semi-transparent membrane, the retina (R.), covered on 



