1873.] 



Dr. J. Jago on Visible Direction. 



215 



Conversely, the parallax of the visual field being noted, we can assign 

 the retinal displacements that have produced them. 



Should undue contraction of any orbital muscle, or discordant con- 

 tractions of the orbital muscles, engender visual parallaxes, we may as 

 safely judge from these parallaxes of the retinal displacements that 

 must have been induced, as if they had been due to manipulation of the 

 eyeball. 



In these summarized conclusions we have the means of solving highly 

 important problems in physiological optics. 



It is found that sensation, or the function of responding to objective 

 light, is exclusively resident in the retinal elements of the bacillar layer, 

 but that the visual functions of the retina extend no further ; for it has 

 been evinced in manifold experiments that when the axes of all the 

 pencils of objective light which concur in imaging a picture upon the 

 retina are normals to its surface, any point in the picture may be perceived 

 as lying in successive directions, forming very variable angles with and 

 round about its normal. The retina cannot inform us of the visible 

 direction of any point painted on its surface. 



This being so, there is no alternative but to seek for a solution of the 

 mystery in the structure out of which the retina proceeds, for the pro- 

 perty in question is plainly inherent in the visual nervous apparatus. 



The author recalls that he had long ago pointed out that, though the 

 optic nerve in its orbital course and its fibres in their retinal course 

 are obnoxious to mechanical pressure, no visual sensation can be im- 

 mediately produced by such pressure on nerve-trunks or branches. Sen- 

 sation can only be produced by pressure through the sclerotic by affecting 

 the rods and cones of the bacillar layer, and then only when the flexure of 

 the retina crowds together the internal (as to the eye) ends of the bacillar 

 elements. He cites his former words : — " When we turn in the dark the 

 eyeballs sharply, or even mildly, a couple of white circular rings, brighter at 

 one margin than the other, enclosing a paler area with a central dark spot, 

 flash forth, the diameter extending an augle of several degrees. ..... 



The phenomenon is plainly the result of flexure of the retina where the 

 nerve runs into it, as the eye is pulled round in its socket until it drags 

 upon the nerve ; and it is to be noted that it is again where the inner re- 

 tinal elements are squeezed laterally that the phenomenon is disclosed." 

 The absence of the tough and dense sclerotic where the nerve penetrates 

 it, as well as of the choroid, indicates how readily the nerve must yield to* 

 the slightest traction. 



In these previously recorded facts the author feels assured that he 

 had, unwittingly, provided himself with a key to the secret of visible- 

 direction. 



Tor it has been shown by diversified experiments that whenever there 

 is a parallax in visible direction it is accompanied with a displacement of 

 the base of the optic nerve in the same direction, that is to say, with trac- 



