CHAP, in.] SIGHT. 1183 



t 



730. 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 aque- 

 duct, 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, and that in a corresponding way. Atropin 

 paralyses it, so that the eye remains adjusted for far objects ; and 

 physostigmin throws the eye into a condition of forced accommo- 

 dation 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 

 paralyses, 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. For instance, we have no clear evidence that 

 the mechanism is, like that of the pupil, a double one ; when we 

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

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

 ciliary muscle, and, so far as we know at present, the return of the 

 suspensory ligament and other parts is simply the passive result 

 of the cessation of the contraction of the ciliary muscle. We have 

 no evidence of antagonistic impulses passing along the long 

 ciliary nerves for instance, and undoing the work of the previous im- 

 pulses. It has, however, been stated that when the eye is brought 

 in forced accommodation for near objects by the action of physo- 

 stigmin, stimulation of the long ciliary nerves abolishes or dimi- 

 nishes the accommodation, bringing about a flattening of the 

 anterior surface of the lens by inhibiting the previous contraction 

 of the ciliary muscle. We may add that, were the change from 

 near to far a mere passive relaxation of a previous contraction we 

 should, judging from our experience of ordinary muscular con- 

 tractions, 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 ; 



