SENSE OF SIGHT. 611 



nearly flat that it throws no perceptible shadow under an extreme late- 

 ral illumination ; but in normal eyes, as a rule, the portion immediately 

 surrounding the pupil is sufficiently prominent to throw a distinct 

 shadow ; and if the source of illumination be not more than one milli- 

 metre in advance of the edge of the cornea, this shadow may extend 

 even to the opposite border of the iris. 



When the pupil dilates, the central prominence of the iris of course 

 diminishes, or even disappears altogether; but, according to Helmholtz, 

 the pupillary border of the iris hardly separates from the anterior face 

 of the lens, even in the most complete dilatation obtainable by bella- 

 donna. 



An important portion of the structure of the iris is formed by its 

 muscular fibres. These are arranged in two sets, both of which consist 

 of unstriped fibres, namely, the sphincter and the dilator muscles of the 

 pupil. 



The sphincter pupillse is composed of bundles of muscular fibres, 

 situated at the pupillary margin of the iris, and circularly disposed, in 

 such a manner that their contraction has the effect of diminishing the 

 orifice of the pupil, while their relaxation allows of its enlargement. 

 When the sphincter is in a state of moderate contraction, the remaining 

 non-contractile portions of the iris are thrown into radiating folds, 

 which can be readily seen, under the influence of ordinary daylight, 

 extending from the pupillary margin for one-third or one-half the dis- 

 tance toward its outer border. 



The dilator pitpillde, which consists of radiating muscular fibres, is 

 much more difficult of demonstration, and its existence in man con- 

 tinued to be a matter of uncertainty, even after it was known to be 

 present in the lower animals. It has, however, been described by so 

 many independent observers, that there can be no doubt of its forming 

 a normal part of the muscular apparatus of the iris. Its fibres are 

 interwoven with those of the sphincter at the pupillary margin, and 

 extend thence in a diverging direction toward the attached border ; 

 either as isolated bundles running between the bloodvessels (Briicke, 

 Kolliker), or as a very thin, continuous sheet of fibres, covering the 

 whole posterior surface of the iris, immediately underneath its pig- 

 mentary layer (Henle, Iwanoff). According to Kolliker, the iris also 

 contains elements analogous to the fibres of elastic tissue, which may 

 thus assist the action of the dilator. 



Notwithstanding the acknowledged existence of both these muscles, 

 and their evident physiological association with each other, the action 

 of the sphincter is much the most prominent and the most clearly 

 understood. It is this muscle which contracts under the influence of 

 light falling upon the retina, causing contraction of the pupil, and which 

 relaxes when the stimulus is withdrawn, causing dilatation. The con- 

 traction of the pupil is therefore, for the most part, an active movement; 

 its dilatation a passive one. Division of the oculomotorius nerve, loss 

 of sensibility in the retina, opacity of the crystalline lens, or insensi- 



