THE VEKTEBRATE EYE. 



121 



an image of ah c will be tbrovva on a screen or on a 

 retina at a' V c'. The image, it will be observed, is 

 necessarily reversed. This is the form of eye which 

 we possess ourselves: it is, in fact, a camera obscura. 

 It is that of all the higher animals, of most molluscs, 

 the ocelli of insects, etc. 



Fig. 77, taken from Helmholtz, will give an idea of 

 the manner in which we see. 



Fig. 77. — (?, Vitreous humor; Z., lens; W, aqueous humor; c, ciliary procesB; d^ 

 optic nerve ; e e, suspeuBory lipament ; k k, hyaloid membrane \ f t, h h, coroea ; 

 g g, choroid ; i, retiua ; 1 1, ciliary muscle ; mf, nf, sclerotic coat ; p p, iris ; s, 

 the yellow spot. 



The eyeball is surrounded by a dense fibrous mem- 

 brane, the siderotic coat, or white of the eye, mf, nf, which 



