22 THE NERVOUS SYSTEM. 



pupil is sufficiently prominent to throw a distinct shadow ; and if the 

 source of illumination be not m6re than one millimetre in advance of 

 the edge of the cornea, the shadow may extend even to its opposite 

 border. 



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

 diminishes, or disappears ; 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 belladonna. 



The muscular fibres of the iris are arranged in two sets, namely, the 

 sphincter and dilator muscles of the pupil. 



The sphincter pupillse is composed of circular fibres, situated at the 

 pupillary margin of the iris, in such a manner that their contraction 

 diminishes the orifice of the pupil, while their relaxation allows its 

 enlargement. When the sphincter is in a state of moderate contrac- 

 tion, the remaining non-contractile tissues are thrown into radiating 

 folds, which extend from the pupillary margin for one-third or one- 

 half the distance toward the outer border of the iris. 



The dilator pupillae, which consists of radiating fibres, is more dif- 

 ficult of demonstration, and its existence in man continued to be a 

 matter of uncertainty, after it was known to be present in 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 muscu- 

 lar apparatus of the iris. Its fibres are interwoven with those of the 

 sphincter at the pupillary margin, and thence diverge toward the 

 attached border of the iris, either as isolated bundles running between 

 the blood-vessels (Briicke, Kolliker), or as a very thin, continuous 

 sheet on the posterior surface of the iris, beneath its pigmentary layer 

 (Henle, Iwanoff ). According to Kolliker, the iris also contains ele- 

 ments analogous to the fibres of elastic tissue, which may assist in its 

 dilatation. 



The pigmentary layer, which is continuous, over the inner surface 

 of the choroid, the ciliary processes, and the posterior surface of the 

 iris, is called the system of the uvea, from its resemblance to the skin 

 of a purple grape separated from its stem, the opening of the mem- 

 branous sac at the point of detachment representing the orifice of the 

 pupil. Owing to the existence of this layer, no light can penetrate 

 the eyeball except through the pupil ; and rays which have reached the 

 retina at any point are arrested there, and prevented from dispersion 

 over other parts. 



Aqueous Humor and Vitreous Body. The cavity of the eyeball is 

 divided, by the transverse partition of the iris, into two portions an 

 anterior and posterior. The portion in front of the iris, called the 

 " anterior chamber," is filled with a colorless, transparent watery fluid, 

 the aqueous humor. This fluid serves to maintain the internal tension 

 of the eyeball, and to allow of changes in the iris and crystalline lens, 



* Optique Physiologique. Paris, 1867, p. 20. 



