GENERAL ORGANOLOGY. 



155 



that the incidence of light must be regulated ; that means 

 for nutrition (the choroid coat) and for protection (sclerotic 

 coat) must be provided. If all these parts are present a 

 structure results such as is found in the squid and in th<* 

 vertebrates (Fig. So). 



NO ro 



FIG. So. Horizontal section through the human eye. (After Arlt, from Hatschek.) E, epithe- 

 lium of the cornea (conjunctiva) ; C, cornea ; vA, anterior chamber of the eye ; /, iris ; 

 //.-/, posterior chamber of the eye: Z, zonula zmnii : Os, ora serrata ; Sc, sclerotic coat; 

 C/i, choroidea; R, retina ; /.Jpapilla of optic nerve; m, macula lutea, area of most distinct 

 viMuii; I'O, sheath of the optic nerve ; NO, optic nerve ; C, arteria centralis ; Cc, corpus 

 ciliare ; L, lens; Cv, vitreous body. 



The Eye of the Vertebrates. The eye of the verte- 

 brates is a body, in general, approximately spherical, whose 

 surface is formed by a firm membrane. Over the greater part 

 of the circumference is an opaque fibrous or cartilaginous 

 covering, called the sclerotic coat, or sclerotiea; it is trans- 

 parent only in the most anterior part, and here it forms by 

 its greater convexity a projecting portion like the crystal of 



