THE SENSES 793 



causes temporary paralysis of accommodation and dilatation 

 of the pupil. When the third nerve is divided, the pupil 

 dilates ; it dilates still more when atropia is administered 

 after the operation. Dropped into one eye in small 

 quantity, atropia only produces a local effect ; the pupil 

 of the other eye remains of normal size, or somewhat con- 

 stricted on account of the greater reflex stimulation of its 

 third nerve by the greater quantity of light now entering the 

 widely-dilated pupil of the atropinized eye. Even in the 

 excised eye the effect of the drug is the same. Introduced 

 into the blood, atropia causes both pupils to dilate. Other 

 mydriatic, or pupil-dilating drugs, are cocaine, daturine, and 

 hyoscyamine. Eserine, pilocarpine, and morphia are the chief 

 myotics, or pupil-constricting substances. They also cause 

 spasm of the ciliary muscle, and inability to accommodate 

 for distant objects. The work of the mydriatics can be 

 undone by the myotics. Thus the dilatation produced by 

 atropia is removed by pilocarpine. The explanation of the 

 action of these drugs is that the mydriatics paralyze the 

 third nerve, and stimulate the dilator nerve - fibres of 

 the iris, while the myotics paralyze the dilators and stimu- 

 late the third. Nicotine, which ultimately causes constric- 

 tion of the pupil, does so by paralyzing the cells on the 

 course of the dilating fibres in the superior cervical ganglion. 



Inward rotation of the eyes is associated with contraction of the 

 pupil, and the contraction that occurs during sleep is thus explained, 

 for in sleep the eyes converge to some extent. When the pressure in 

 the anterior chamber is diminished, as by tapping the aqueous humour 

 through the cornea, contraction of the pupil occurs ; and stimulation 

 of the sympathetic has now a far smaller dilating effect than usual. Re- 

 moval of the cornea narrows the pupil, partly by occasioning direct 

 stimulation of the sphincter pupillae, partly by abolishing the pressure 

 of the aqueous humour. The attached (ciliary) border of the iris 

 then bulges forward, and the pupil becomes smaller. On the other 

 hand, an increased pressure in the anterior chamber forces back the 

 ciliary border of the iris, and causes mechanical dilatation of the pupil. 



Functions of the Iris. In vision, the iris performs two 

 chief functions : (i) It regulates the quantity of light allowed 

 to fall upon the retina. The larger the aperture of a lens, 

 the greater is its collecting power, the more light does it 

 gather in its focus. In the eye, the area of the pupil 

 determines the breadth of the pencil of light that falls upon 



