80 THE ANATOMY OF VERTEBKATED ANIMALS. 



connective tissue ; so that the primitive cavity of the diver- 

 ticuhim, which, of course, communicates freely vd.th that of 

 the anterior cerebral vesicle, is obliterated. The broad end of 

 the diverticulum acquii-ing a spheroidal shape, while its 

 jjedicle narrows and elongates, the latter becomes the optic 

 nei-ve, while the former, surrounding itself with a strong 

 fibrous sclerotic coat, remains as the posterior chamber of 

 the eye. The double envelope, resulting from the folding 

 of the wall of the cerebral optic vesicle upon itseK, gives 

 rise to the retina and the choroid coat : the plug, or in- 

 growth of connective tissue, gelatinises and passes into the 

 vitreous humour, the cleft by which it entered becoming obli- 

 terated. 



Even in the higher Vertebrata the optic nerve is, at first, 

 connected exclusively with the vesicle of the third ventricle, 

 and makes no decussation with its fellow. But by degrees 

 the roots of origin of each nerve extend over to the opposite 

 side of the brain, and round the thalamus, to the mesen- 

 cephalon on that side, and the trunks of the two nerves 

 become intermixed below the third ventricle, in a close and 

 complicated manner, to form a chiasma. 



In Amphioxus and Myxine, the eyes are very imperfectly 

 developed, appearing to consist of little more than a rudi- 

 mentaiy lens imbedded in the pigment, which encloses the 

 termination of the optic nerve ; and in Myxine, this rudi- 

 mentary eye is hidden by miiscles and integument. It ap- 

 f)ears doubtful whether in these fishes, and in the Lampreys, 

 the eye is developed in the same way as in other Vertebrata. 



In all other Vertebrata, the eyes have the typical structure, 

 though sometimes, as in the Blind-fish (Amblyopsis) and the 

 Mole, they have no fimctional importance. In the Ichthy- 

 opsida and Sauropsida, but not in Mammalia, the sclerotic 

 is often partially ossified, the ossification usually forming a 

 ring around its anterior moiety. It becomes enormoxisly 

 thickened in the Cetacea. 



Except in Am2yhiox2is and the Myxinoid fishes, the eye- 

 ball is moved by six muscles ; of these, four, proceeding 

 from the interior of the orbit to the periphery of the eye- 



