1050 VISION. 



of the pigment and cones. Engelmann 1 found that if only one eye 

 of a frog was exposed to light, both eyes showed the forward position 

 both of pigment and cones. After removal of the brain, the changes 

 were limited to the eye acted on. Engelmann concluded that there 

 were centrifugal " retino-motor " fibres in the optic nerve. The same 

 position as that due to light was also brought about by means of 

 strychnin, and by the application of tetanising currents to the eye. 

 Engelmann also found that the same forward movement occurred on 

 stimulation of the skin. A frog with the anterior half of the body pro- 

 tected from light, and the posterior half exposed, showed the light position, 

 especially of the pigment. A. E. Fick 2 objected that variations in the 

 position of pigment occurred in dark frogs ; that results similar to those 

 of Engelmann occurred after section of the optic nerve ; and that the 

 experiment showing the influence of skin stimulation was vitiated by 

 the interference with respiration caused by the covering of the anterior 

 half of the animal. Tick only observed the position of the pigment. 

 Nahmmacher 3 observed the position of the cones in a large number of 

 frogs. He used chemical stimulation of various parts with and without 

 previous section of the optic nerve. After stimulation of the optic 

 chiasma, he found forward position in 82'3 per cent, of the cases in which 

 the optic nerve had not been divided, and only 18*4 per cent, in those 

 with divided nerve. Further evidence in favour of centrifugal influence 

 was derived by Engelmann from the electrical behaviour of the two eyes, 

 and the weight of evidence seems to be distinctly in favour of the 

 existence of " retino-motor " fibres in the optic nerve. 



There seems now to be definite anatomical evidence of the existence of such 

 centrifugal fibres, which pass from the nerve fibre layer directly into the inner 

 reticular layer, and then either terminate in free endings near the border of the 

 inner nuclear layer (Ramon y Cayal), or join the cells in that layer (Dogiel). 



Microchemistry. Certain other differences between a retina which 

 has, and one which has not, been exposed to light have been noted. The 

 reaction of the former is said to be acid, that of the latter alkaline. Birn- 

 bacher 4 found the illuminated retina stained yellow with eosin, while the 

 dark retina was coloured an intense pink. With Biondi-stain the cones of the 

 former were coloured green, those of the latter yellow. Dor 5 has tried the 

 effect of coloured light ; with red light he found a difference in the staining of 

 rods and cones ; while with blue light there was no such difference. The rod 

 and cone nuclei have been described as differing greatly in the illuminated and 

 non-illuminated retina, being round and deficient in chromatin in the former, 

 elongated and with abundant chromatin in the latter case (Mann, 6 Pergens 7 ). 



Retinal currents. The electrical condition of the retina, and its 

 modifications on the action of light, have been much investigated, but 

 their exact nature is still undetermined. When the optic nerve is 

 connected directly with the retina, a current of rest is indicated; if 

 connection is made with the outer side of the retina, i.e. with the rods, 

 the nerve is positive, the retina negative ; if, on the other hand, the 



1 Arch.f. d. ges. PhysioL, Bonn, 1885, Bd. xxxv. S. 498. 



2 Vrtljschr. d. naturf. Gesellsch. in Zurich, 1890, S. 83 ; and Arch. f. Ophth., Bd. 

 xxxvii. Abth. 2, S. 1. 



3 Arch.f. d. ges. PhysioL, Bonn, 1893, Bd. liii. S. 375. 



4 Arch.f. Ophth., 1894, Bd. xl. Abth. 5, S. 1. 



5 Ann. d'ocul., 1896, tome cxv. p. 443. 



B Jonrn. Anat. and PhysioL , London, 1895, vol. xxix. p. 105. 



7 Ann. Soc. roy. d. sc. mdd. et not. de Bruxelles, 1896, tome v. p. 389. 



