SIGHT. 165 



(2) The vitreous body (or humor) is a semifluid gelatinous 

 substance which fills the posterior chamber and constitutes about 

 four-fifths of the bulk of the eye. It is quite transparent, and 

 acts to maintain the tension of the eyeball, and as a refracting 

 medium through which the light reaches the retina. 



What is the pupil? 



The pupil is the aperture through which light enters the dark 

 chamber of the eye. It is a circular opening in the iris. 



What is the iris? 



It is a curtain of muscular tissue placed vertically in front of 

 the lens. The fibres of the muscular tissue are both circular and 

 radiating, so that they serve to decrease and increase the size of 

 the pupil as one or the other set of fibres acts. It has .a pigment- 

 layer upon the inner surface, which is continued from the choroid, 

 and upon the amount of the coloring-matter depends the " color of 

 the eyes." 



What nervous action controls the ciliary muscles ? 



Contraction or dilatation of the pupil is a reflex act, and the 

 afferent stimulus is carried through the optic nerve and the motor 

 through the third cranial nerve, acting from a centre just beneath the 

 aqueduct of Sylvius and the corpora quadrigeniina. The increase 

 in the amount of light which reaches the retina causes a contrac- 

 tion of the pupil, and a decrease is followed by dilatation. Aside 

 from this, the needs of the eye regulate the amount of light ; thus, 

 for near work the pupil contracts, and dilates when the eye is 

 focused upon a distant object. 



What other influences control the iris ? 



The pupil is controlled also through the sympathetic, and the 

 fifth nerve through the connection of the third and fifth nerves 

 with the ciliary ganglion of the sympathetic system. Drugs are 

 active also, both locally and internally, in controlling the action of 

 the iris without reference to the reflex fibres : atropine both locally 

 and internally dilates the pupil ; opium internally, and eserine locally 

 contract it. 



What is accommodation in vision ? 



The eye is able to see objects close at hand and at a distance 

 with equal clearness, though perhaps not with equal regard for the 



