90 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



through the opening in which the rays of light enter the eye to fall 

 on the terminal segments of the retinal cells (fig. 87). 



In the more highly developed forms of this type of eye, especially 

 in the Vertebrate eye, the peripheral portion of the optic nerve 

 spreads out so as to form a cup-shaped nervous membrane, the retina, 

 placed immediately behind the refractile media and surrounded by a 

 vascular pigmented membrane, the choroid. The choroid, again, is 

 surrounded by a tough supporting membrane composed of fibrous 

 connective tissue, and known as the sclerotic, which is continued over 

 the anterior part of the eye, i.e., that part through which the light 

 passes, as a thinner transparent membrane. Of the refractile media 

 which are placed behind the cornea and fill the cavity of the optic 



bulb, viz., the aque- 

 ous humour, the lens 

 (fig. 88 L\ the vitre- 

 ous humour (6-7), the 

 lens is the most 

 powerful. Grasped 

 by the thickened 

 muscular anterior 

 part of the choroid 

 (the ciliary body (Cc) 

 and ciliary processes), 

 the peripheral part 

 of its anterior face is 

 covered by a forward 

 continuation of the 

 choroid, the iris (Jr\ 

 which, as a ring-like 

 contractile border, 



forms a kind of diaphragm perforated by a central contractile opening, 

 the pupil, through which the light enters the eye (fig. 88). The 

 reversed image which is formed in the hinder part of the Vertebrate 

 eye on the cup-shaped retina has a very considerable brilliancy and 

 definition. 



The eyes of many Cephalopods may be looked upon as a modifica- 

 tion of this type of eye. In the eye of Nautilus the lens is absent, 

 and the light enters through a small opening. In this case a 

 reversed, but not brilliant, image is formed on the retina placed on 

 the hinder wall of the eye. 



To enable the eye to see clearly objects in different directions and 



Rz 



FIG. 87. Transverse section through the simple eye of a 

 beetle larva (.partly after Grenacher). CL, corneal lens ; 

 Gk, the subjacent hypoclermis cells, the vitreous humour 

 of Authors ; P, pigment in the peripheral cells of the lat- 



i ter ; Jfo, retinal cells. St, cuticular rods of the latter. 



