868 MOVEMENTS OF THE PUPIL. [BOOK in. 



543. The nervous mechanism of accommodation. The ciliary 

 muscle which brings about accommodation is governed in this 

 action by fibres which may be traced, through the short ciliary 

 nerves and ciliary ganglion, along the third nerve, to a centre 

 which lies (in dogs) in the extreme front of the floor of the 

 aqueduct, or rather perhaps in the extreme hind part of the floor 

 of the third ventricle, and which is especially connected with the 

 extreme front of the nucleus of, and so with the most anterior 

 bundles of the root of, the third nerve. As we have already 

 said stimulation of this centre, or of the third nerve, or of the 

 short ciliary nerves, leads to a contraction of the ciliary muscle 

 and to accommodation for near objects. 



This nervous mechanism, unlike that for the pupil, is under 

 the command of the will, though the will needs to be assisted 

 by visual sensations ; 'it is moreover only brought into play by 

 the direct action of the will ; we are not led to accommodate by 

 any other influence than the desire to see distinctly near or far 

 objects. The mechanism may, however, be affected by the local 

 action of drugs. Such drugs as atropin and physostigmin which 

 have a special action on the pupil, also affect the mechanism of 

 accommodation. Atropin paralyzes it, so that the eye remains 

 adjusted for far objects ; and physostigmin throws the eye into a 

 condition of forced accommodation for near objects. This double 

 action has been explained by the supposition that, by acting on 

 the muscular fibres, or on the nerve endings, or on both, atropin 

 inhibits the contraction of or paralyzes, while physostigmin 

 throws into contraction or augments the contraction of the 

 ciliary muscle. But the phenomena, on further inquiry, are 

 found to be more complicated than they appear to be at first 

 sight. There are also other facts which indicate that our knowl- 

 edge of the mechanism of accommodation is far from being com- 

 plete. For instance, so far as we know at present, when we pass 

 from accommodation for a near object to that for a far object, we 

 simply 4 let go ' the previous effort ; we cease to contract the 

 ciliary muscle, and the return of the suspensory ligament and 

 other parts is simply the passive result of the cessation of the 

 contraction of the ciliary muscle. If, now the change from near 

 to far be such a mere passive relaxation of a previous contrac- 

 tion we should, judging from our experience of ordinary mus- 

 cular contractions, expect the time taken up by it to be greater, 

 or at least not less than the time taken up by the change from 

 far to near ; but as a matter of fact it is very much shorter, 

 indeed the act is an exceedingly rapid one. 



544. There remains a word to be said concerning the con- 

 striction of the pupil which takes place when the eye is accom- 

 modated for near objects, and when the pupil is turned inwards 

 (the two being closely allied, since the two eyes converge to see 

 near objects), and the return to the more dilated condition when 



