1078 VISUAL SENSATIONS. [BOOK in. 



garded as a whole, with its central as well as its peripheral parts, 

 by referring to certain defects of vision due to changes in the eye 

 itself. The eye may fall into such a condition, that the mind can 

 only appreciate, and that to a varying degree, the difference 

 between light and darkness; the mind is aware that the retina 

 (or it may be part of the retina) is being stimulated to a less 

 or greater degree, but cannot perceive that one part of the retina 

 is being stimulated in a different way from another part ; a 

 sensation of light is excited, but not a set of visual sensations 

 corresponding to the sets of pencils of luminous rays, which, 

 reflected, or emanating from external objects in a definite order, 

 are falling upon the eye. The eye again may fall into another 

 condition, in which such sets of visual sensations are excited, but^ 

 on account of dioptric imperfections or for other reasons, the 

 several sensations are not adequately distinct ; the mind is aware 

 through the eye of the existence of ' things/ but cannot adequately 

 recognize the characters of those things ; the visual images are 

 blurred and indistinct. And a large number of gradations are 

 possible between the extreme condition in which only those 

 objects which present the strongest contrast with their surround- 

 ings are visible, to a condition which only just falls short of normal 

 vision. Imperfections of this kind, of varying degree, may result 

 from failure not in the peripheral apparatus, not in the retina, or 

 optic nerve or other parts of the eye, but in the central apparatus ; 

 the retinal image may be sharp, the retina and the optic fibres 

 may be duly responsive, but from something wrong in some part 

 or other of the brain, the visual sensations excited by the visual 

 impulses may fail in distinctness, and that in varying degree : 

 imperfections of vision whether of central or peripheral origin, in 

 which visual sensations fail in distinctness are generally spoken of 

 under the not wholly unexceptionable name of amblyopia. 



If one optic nerve be divided, total blindness of one eye will 

 result ; but if one optic tract be divided, it follows from what has 

 been said above, that half-blindness in the corresponding halves of 

 both eyes will result. If, for instance, the right optic tract (Fig. 

 131, Op. T.) be divided, the left visual fields of both eyes will be 

 blotted out. The same condition will be brought about by failure 

 in the optic tract at its central ending, provided of course the 

 mischief be confined to the ending of the one tract. Such a half- 

 blindness or half- vision is spoken of as hemianopsia, or hemianopia 

 or hemiopia; the words left and right are generally used in 

 reference to the visual field ; thus left hemianopsia is the blotting 

 out of both left visual fields, through failure of the right optic 

 tract. 



If instead of the whole optic nerve being divided, certain 

 bundles only were cut across, partial blindness would be the 

 result, a portion of the visual field would be blotted out ; and 

 mischief limited to a few bundles of one optic tract would lead 



