Chap, ii.] THE BRAIN. 789 



cils 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 sensa- 

 tions 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 sur- 

 roundings are visible, to a condition which only just falls short 

 of normal vision. Imperfections of this kind, of varying de- 

 gree, 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 distinct- 

 ness, and that in varying degree : imperfections of vision 

 whether of central or peripheral origin, in which visual sensa- 

 tions 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. 138, 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 Jiemianopia 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 in that eye would be the result, a portion of 

 the visual field of that eye would be blotted out. If not the 

 optic nerve but the optic tract were so treated, mischief 

 limited to a few bundles of one tract would lead to correspond- 

 ing blots in the corresponding halves of the visual fields of both 

 eyes. Further, an affection of half the retina or of a limited area 

 in the retina might occur of such a character as to lead not to 

 complete, but to partial blindness, to a hemi-amblyopia or to a 

 partial amblyopia. The part of the retina so affected might be 

 central, or peripheral, and might be a quadrant, or any patch of 

 any size, form and relative position. And we may further im- 



