OPTIC NERVES. 4:1 



the retina ; and two pathological cases have lately been re- 

 ported by Drs. Keen and Thomson, of Philadelphia, which 

 go to show that this is the fact, and which illustrate certain 

 interesting points in connection with the decussation of the 

 nerves. One was a case of gunshot-wound of the head with 

 severe injury of the brain-substance. This case presented, 

 immediately after the injury, unconsciousness and partial 

 paralysis of the right arm and right leg, which lasted two or 

 three months. About a year after, the paralysis had almost 

 entirely disappeared, but the memory was somewhat im- 

 paired. Upon careful examination of the eyes, it was ascer- 

 tained that the field of vision was divided in each eye by a 

 vertical line passing through its centre. In the right eye, the 

 inner half of the retina, beginning at the inner border of the 

 macula lutea, was entirely insensible to light. In the left 

 eye, the outer half of the retina, beyond the macula, was in- 

 sensible to light. 'No pathological appearances were ob- 

 served on examining the retinae with the ophthalmoscope. 

 The second case, reported by Dr. "W. Thomson, presented 

 the same condition following partial hemiplegia, the result of 

 sunstroke. The peculiar affection of vision in these cases, 

 called hemiopsia, especially as illustrated in the first case, 

 reported by Dr. Keen, can be explained by assuming the fol- 

 lowing as the course of the decussating fibres of the optic 

 tracts : From the left side of the encephalon, visual fibres 

 pass to the right eye, supplying the inner mathematical half 

 of the retina, from a vertical line passing through the macula 

 lutea. Yisual fibres also pass to the left eye, supplying the 

 outer half of the retina, beginning at the macula lutea. The 

 macula lutea, then, and not the point of entrance of the optic 

 nerve, is in the line of division of the visual field. The 

 outer half of the left and the inner half of the right retina 

 are supplied by fibres from the left side ; and the outer half 

 of the right and the inner half of the left retina are sup- 

 plied from the right side. Though this anatomical arrange- 

 ment has not been actually demonstrated, it is rendered ex- 



