MUSCLES OF THE IRIS. 577 



relaxes because it has lost the stimulus reflected from the fragile 

 retina. 



The nerves supplying the dilator muscle seem to be derived 

 from the sympathetic, for when the sympathetic in the neck is 

 cut, the pupil remains permanently contracted. These fibres are 

 supposed to take origin in the gray matter of the cervical spinal 

 cord. The sympathetic also supplies the muscles in the walls of 

 the vessels, and thus controls the amount of blood going to the 

 iris. Though the variation in blood supply may cooperate in 

 causing dilatation, it cannot be the only cause, as the widening 

 of the pupil may be caused in a bloodless eye. 



The nerve mechanism by which the sphincter muscle is made 

 to contract is quite distinct, and more definitely understood. Its 

 contraction is a reflex act, the stimulus of which starts in the 

 retina and travels along the optic nerve as an afferent channel to 

 the corpora quadrigemina, where there is one centre governing 

 the contraction of both irides. The efferent impulses are sent 

 along the third nerve to the lenticular ganglion, and thence by 

 the short ciliary nerves to the eyeball. 



When we accommodate for near objects three muscles act in 

 unison, so we say their movements are " associated " with one 

 another. The voluntary effort that causes the ciliary muscle to 

 relax the suspensory ligament, makes the sphincter of the iris 

 contract, and also stimulates the internal rectus to move the eye 

 inward. The voluntary nerve centre must be in intimate rela- 

 tion to the reflex centre, which keeps up the tonic action of the 

 sphincter iridis. 



We have then central nerve governors for the ciliary and iris 

 movements. The ciliary muscle and sphincter of the pupil are 

 together stimulated by the will, and the sphincter alone is excited 

 by means of a centre which reflects the stimulus arriving from 

 the retina by the optic nerve along the branches of the third 

 nerve. The dilator of the pupil, if a muscle, is also kept in 

 gentle tonic action by the impulses sent from the spinal marrow, 

 via the sympathetic, with the vasomotor impulses ; but some think 

 that the blood supply and tissue elasticity explain the dilatation. 



From the facts (1) that reflex contraction of the pupil may be 

 49 



