430 Mr. J. L. Tupper on the Centre of Motion in the Eye. [June 18, 



or somewhere in the visual line behind the nodal point, a position 

 which agrees with that assigned to the centre of motion by the preceding 

 analysis. 



(2nd) The dissected organ exhibits an asymmetrical attachment of 

 the recti muscles, so that a vertical plane cutting these attachments 

 is further from the external than from the internal margin of the 

 cornea. 



The circumference of this plane would be a circle, and the attachment 

 of the globe's suspensory ligament, that resists the backward traction of 

 these muscles, is found also to be a circle parallel to, and one line further 

 back than, the former circle. The latter may be considered the base of 

 a cone, whose vertex is the optic foramen, in the surface of which cone 

 the recti muscles are situate. The base is therefore kept in equipoise 

 by the symmetrical arrangement of the contracting muscles behind 

 and the resisting suspensory ligament in front ; so that the contrac- 

 tion of a single rectus, as it draws back the ligament on one side, 

 increases its forward traction on the other side, and moves any two 

 opposite points of the cone's base equally in opposite directions, or 

 rotates it on its centre, a centre which is thus the anatomical centre of 

 motion. 



But however the recti are situate (and act) symmetrically with the base 

 of this cone, the base is oblique with respect to the cornea (not at right 

 angles to its axis), and consequently its centre will be on one side of the 

 cornea's axis ; and again, since the cone's base is further from the outer 

 than from the inner margin of the cornea, its centre will be outside 

 the cornea's axis. Now that part of the visual line where the preceding 

 experiments have placed the centre of motion is outside the cornea's axis, 

 while the base of the cone, whose centre has thus proved to be the ana- 

 tomical centre of motion, is found to pass through the visual line -^ of an 

 inch behind the cornea, exactly in accordance with the results of the 

 experiment with sighted radii of a circle. 



Lastly, the obliquity of the cone's base with the base of the cornea 

 proves to be a consequence of the hitherto unexplained want of lateral 

 symmetry in the attachment of the recti muscles, thus explained as 

 a most important means of adjusting the eye's visual line to the ob- 

 ject ; while some further peculiarities in the insertion of the recti, 

 demonstrated in the author's dissections, conspire to attain the same 

 end. 



The author's thanks for valuable assistance are due to Mr. J. Salter, 

 F.R.S., to Mr. H. G. Howse, Demonstrator of Anatomy to Guy's Hos- 

 pital, and to the Rev. Geo. P. Wright, of Overslade, Rugby. 



