THE EYE. 73 



nea, growing out into a fold above and below, results in the 

 formation of the ej'elids, and the segregation of the integu- 

 ment which they enclose, as the soft and vascular conjunctiva. 

 The pouch of the conjunctiva very generally communicates, 

 oy the lachrymal duct, with the cavity of the nose. It may 

 be raised, on its inner side, into a broad fold, the nictitating 

 membrane, moved by a proper muscle or muscles. Special 

 glands -the lachrymal externally, and the Harderian on the 

 inner side of the eyeball — may be developed in connection 

 with, and pour their secretion on to, the conjunctival mucous 

 membrane. 



The posterior chamber of the eye has a totally distinct ori- 

 gin. Very early, that part of the anterior cerebral vesicle 

 which eventua.ly becomes the vesicle of the third ventricle, 

 throws out a diverticulum, broad at its outer, and narrow at 

 its inner end, which applies itself to the base of the integu- 

 mentary sac. The posterior, or outer, wall of the diverticulum 

 then becomes, as it were, thrust in, and forced toward the op- 

 posite wall, by an ingrowth of the adjacent connecrtive tissue ; 

 so that the primitive cavity of the diverticulum, which, cf 

 course, communicates freely with that of the anterior cerebral 

 vesicle, is obliterated. The broad end of the diverticulum ac- 

 quiring a spheroidal shape, while its pedicle narrows and elon- 

 gates, the latter becomes the optic nerve, while the forme r, 

 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 ves- 

 icle upon itself, gives rise to the retina and the choroid coat: 

 the plug, or ingrowth of connective tissue, gelatinizes aiid 

 passes into the vitreous humor, the cleft by which it entered 

 becoming obliterated. 



Even in the higher 'Vertebrata the optic nerve is, at firs t, 

 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 mesencepha- 

 lon on that side, and the trunks of the two nerves become in- 

 termixed below the third ventricle, in a close and complicated 

 manner, to form a chiasma. 



In Amphioxus and Myxine, the eyes are very imperfeclly 

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

 tary lens imbedded in the pigment, which encloses the termi- 

 nation of the optic nerve ; and, in Myxine, this rudimentary eye 

 is hidden by muscles and integument. It appears doubtful 

 4 



