VISION. 569 
ALTERATIONS IN THE SIZE OF THE PUPIL. 
The pupil varies in size according as the iris is in a greater 
or less degree active. All observers are agreed that the circu- 
lar fibers around the pupillary margin are muscular, forming 
the so-called sphincter of the iris; but great differences of opin- 
ion still exist in regard to the radiating fibers. It is thought 
by many that all the changes in the iris may be explained by 
the elasticity of its structure without assuming the existence 
of muscular fibers other than those of the sphincter; thus a 
contraction of the latter would result in diminution of the pu- 
pillary aperture, its relaxation to an enlargement, provided the 
rest of the iris were highly elastic. 
The conclusions in regard to the innervation of the iris rest 
largely upon the results of certain experiments which we shall 
Brain above medulla 
os ~, 
Dilator centre—-- 
Sympathetic nerve to 
radiating fibres 
Spinal dilator centre —| 3 
Fig. 413,—Diagram to illustrate innervation of the iris. Dotted lines indicate general func- 
tional connection (correlation). Course of impulses indicated by arrows. 
