Chap, hi.] SIGHT. 865 



of contraction, manifested however not in a rhythmic but in a 

 tonic manner, and that like the heart its action may be either 

 augmented or inhibited by nervous impulses ; and we have seen 

 (§ 347) that a similar view may be taken of the actions of the 

 plain muscular fibres of the alimentary canal and of the bladder. 

 According to this view the sphincter of the iris, when removed 

 from all influences, is in a state of tonic contraction, pulling 

 against the radial strain of the elastic tissue of the iris and so 

 maintaining a pupil of a certain size. Under the influence of 

 light falling on the retina, impulses reaching the sphincter by 

 the short ciliary nerves augment its contraction, and narrow the 

 pupil in proportion to their intensity. On the other hand, im- 

 pulses reaching the sphincter from the sympathetic by the long- 

 ciliary nerves inhibit the activity of the sphincter, diminish the 

 force with which it is pulling against the elastic tissue of the 

 iris, and so lead to a widening of the pupil, thus either diminish- 

 ing the constriction which is being caused by the action of light 

 on the retina or otherwise, or, in the absence of all external 

 constricting influences, causing the pupil to become wider than 

 it naturally would when removed from all extrinsic influences 

 whatever. In support of such a view it is pointed out that the 

 muscular tissue forming the sphincter is peculiar, since a slip 

 of it when directly stimulated by a weak interrupted current 

 elongates ; in this respect it shews a further analogy with the 

 heart whose activity may similarly be inhibited by the inter- 

 rupted current. Again, in the extirpated eye, or even in the 

 isolated iris, cold dilates and warmth constricts the pupil, the one 

 relaxing, and the other increasing the contraction of the sphincter. 

 On the other hand the following facts seem to shew that the 

 dilation which we are discussing cannot be simply a matter of 

 inhibition but must be due to the radial pull of some or other 

 contractile elements. By stimulating one ciliary nerve at a 

 time, or by partial direct stimulation of the iris with an electric 

 current, the two electrodes being placed close together on the 

 sclerotic near the outer margin of the iris, an unequal change, 

 in the iris, an uneven pupil may be produced ; and this, since it 

 may be brought about even while the sphincter is contracting, 

 must be due to a radial pull. Further, a mere radial slip of 

 the iris, a slip cut out by two radial incisions carried from the 

 margin of the pupil towards the sclerotic, the slip remaining 

 connected with the sclerotic, may be made by stimulation to 

 shorten. Lastly, since this shortening does not appear, when 

 the hind surface of the iris has been previously sharply brushed, 

 so as to injure it, we may conclude that the layer spoken of 

 above, peculiar though its characters be, is really a radially dis- 

 posed contractile tissue, and by its contraction dilates the pupil. 

 Whatever be the view adopted as to the exact mode of action 

 of the sympathetic there remains the broad fact that the pupil is 



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