CHAP, ix.] 



OF THE SENSE OF SIGHT. 



426 



vertebrate (Fig. 82), that is to say a vitreous globe, receiving 

 in the rear a voluminous nerve, and containing a retina reposing 

 on a pigmentary layer, then divers refractive media, which go 

 from back to front ; the vitreous humour, the crystalline, enchased 

 in a contractible and vascular apparatus, called ciliary process, 

 and protected in front by a coloured contracted screen, the iris. 

 This screen pierced in the centre by a circular opening, the pupil. 



FIG. 81. 



A , transversal and vertical section of an embryon of fish ; c, brain ; a, primitive ocular 

 vesicle ; b, its stem by which it communicates with the medullary tube ; d, dermic layer. 



B, ulterior state, formation of the secondary layer ; p, anterior wall (pigmentary layer) ; r, 

 posterior wall (layer of the retina) of the secondary ocular vesicle ; e, cornean layer (epi- 

 dermis) emitting into the secondary vesicle the crystalline humour ; I, the vitreous body 

 behind. 



merges into a liquid medium, the aqueous humour, which bathes 

 also the anterior face of the crystalline on the one hand, and 

 the posterior face of the transparent cornea on the other. We 

 have not here to describe in all its details the complex structure 

 of the eye of the vertebrates. In sum this retina is formed of 

 bacilla connected behind with cells, into which the optical 

 nervous fibres seem to pass (Fig. 82). 



If we are willing to disregard accessory parts, the eye of man 

 does not differ essentially from that of the arthropods. Each of 

 the bacilla of the human retina is comparable with one of the 

 voluminous optical bacilla of the insects, &c., and the eye of the 

 mammifers may be brought into relation with the compound eye, 



