THE EYE 



161 



pound eyes of the higher groups of both Crustacea and Insects 

 are of a different type. The compound eye is convex and it 

 is made up of a large number of units called ommatidia. The 

 surface of the cuticula is divided into numerous polygonal areas, 

 the "facets," each of which corresponds to an ommatidium. 

 The ommatidium is made up of the following cells: two super- 

 ficial cells which secrete the lens-shaped cuticula; below these, 



FIG. 80. Horizontal section through the right human eye. a-p, Axis of 

 vision; ac, central artery; ah, aqueous humor; b, blind spot; c, conjunctiva; 

 ch, choroid layer of the eyeball; cl, crystalline lens; cmc, circular fibres of the 

 ciliary muscle; c.m.r., radial fibres of the ciliary muscle; co, cornea; cp, ciliary 

 process; cs, canal of Schlemm;/0, fovea centralis; on, optic nerve; os, ora serrata, 

 the anterior limit of the sensory portion of the retinal layer; r, the retina; sc, the 

 sclera; sh, sheath of the optic nerve; vh, vitreous humor. (From Galloway). 



four cells which form an egg-shaped lens, and below these, 

 again, seven or eight sensory cells, so arranged as to form a 

 single sensory unit. Around the whole is a cylindrical curtain 

 of pigment cells. From the sensory cells, nerve fibres pass 

 downward to a deeper ganglion. 



379. The sensory portion of all invertebrate eyes is developed 

 from the epidermis, but the retina of the vertebrate eye and all 

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