MUSCLES OF THE IRIS. 575 



its posterior surface is a dense layer of pigment cells called the 

 uvea, which gives the eye its color. The motions of contracting 

 and dilating the pupil are carried out by smooth muscle fibres. 

 The act of contracting the pupil is performed by a very definite 

 set of fibres forming the sphincter which surrounds the margin 

 of the pupil, while other fibres are said to radiate from the pupil 

 to the attached margin of the iris. The sphincter muscle seems 

 always to be more or less in action, because if it be paralyzed 

 the action of the dilating forces becomes obvious. But the mus- 

 cular character of the dilator has been doubted, from the fact 

 that the fibres have not been satisfactorily demonstrated. Cer- 

 tainly the sphincter seems to be the stronger of the two, for strong 

 electric stimulation causes contraction of the pupil, and shortly 

 after death the pupils dilate. We must assume that the power 

 of the sphincter dies more quickly than that of the dilator, or 

 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 contractions 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 



