22 ADAPTATION OF THE EYE TO DISTANCES. 



posed, be shortened, and the focal point of the rays from distant objects 

 thus be made to impinge on the retina instead of falling short of it, as 

 would be the case if some such adapting power did not exist. According 

 to this view of the action of the muscles, therefore, their effect would be to 

 render distinct the perception of distant objects only: but Hueck gives fur- 

 ther proof that, as was observed by Professor Miiller,* it is in looking 

 at any near objects that we make an active change in the condition of the 

 eye; the vision of distant objects being attended by a comparatively 

 passive state of the organ. The existence, however, of any such alteration 

 in the form of the eye by the action of the recti muscles is quite impro- 

 bable, for, owing to the presence of a yielding mass of fatty tissue at the 

 posterior part of the orbit, a very considerable, and, therefore, quite mani- 

 fest, retraction of the globe must take place before the shortening of the 

 axis of the eye supposed by this hypothesis could be effected. No such 

 retraction, however, is observed to take place during any part of the act of 

 vision. Indeed, as observed by Volkmann,f we do not seem to possess 

 sufficient power over the recti muscles to produce the combined action of 

 all the four at one time ; and except by such combined action, either of ail 

 four, or at least of two opposite ones, retraction of the eye-ball could not 

 be effected. There does not exist any obstacle to the retraction of the eye 

 within the orbit, could the simultaneous action of the recti muscles be 

 induced at will ; this was shewn by Volkmann, who galvanised the third 

 pair of nerves in animals recently slain, and observed well-marked retrac- 

 tion of the eye to ensue in consequence. Another circumstance men- 

 tioned by Volkmann,! to shew how little, if any, share is taken by the recti 

 muscles in adapting the eye to vision at different distances, is that injury 

 of the third pair of nerves, whereby paralysis of three of the recti muscles 

 is produced, is not followed by any material disturbance of the power of 

 adaptation; while, on the other hand, certain pathological conditions 

 sometimes occur, in which, without any alteration in the functions of the 

 muscles of the globe, the eye suffers a temporary impairment, or even per- 

 manent loss of this power. 



In addition to the many proofs already afforded that the action of the 

 iris is not the force concerned in adapting the eye to various distances 

 of vision, and that alterations in the width of the pupil may take place 

 without any corresponding change in the distinctness of objects under view, 

 Hueck states that without altering the direction of the axes of his eyes 

 or the quantity of light admitted, but merely by fixing his attention on 

 a side object, he was able to widen his pupils as much as one half more 

 than their former diameter, without there ensuing any indistinctness of 

 the object towards which the eyes were directed. He observes also that 

 the inefficiency of the iris, in this respect, is demonstrated by the fact that 



* Physiology, p. 1144. t Art. Sehen, in Wagner's Handworterbuch, p. 301. 



Loc. Cit. p. 302. 



