232 THE SPECIAL SENSES 



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

 afferent impulse is carried through the optic nerve, while the 

 motor impulse comes through the third cranial nerve, acting 

 from a centre just beneath the aqueduct of Sylvius and the 

 corpora quadrigemina. An increase in the amount of light that 

 comes to the retina causes a contraction of the pupil, and a 

 decrease is followed by a dilatation. The pupil is controlled 

 by fibers of the sympathetic and fifth cranial nerve which con- 

 nect with the ciliary ganglion. Drugs are active in controlling 

 the action of the iris. Atropine, both locally and internally, 

 dilates the pupil; opium taken internally, and eserin applied 

 locally, contract it. 



If in obtaining an image of an object through a double convex 

 lens the lens be too large, there will be seen around the image 

 formed a halo of prismatic colors. This is called chromatic 

 aberration, and is produced by an unequal refraction of light 

 rays by the peripheral portions of the lens. The unequal 

 refraction results in a dispersion of the light, so that it is broken 

 up into the primary colors. This defect is remedied by putting 

 a shutter in front of the lens, and so limiting the entrance of 

 light to the central portions of the lens, where the index of 

 refraction is constant. In the eye the iris acts as a shutter, 

 thus making the image achromatic, but in some defective eyes 

 where there is considerable fault in the focus of the image on 

 the retina a visible band of color appears. 



Under certain conditions a number pf objects lying within 

 the eye itself become visible. Of these intra-ocular images, the 

 most common are known as muscce volitantes. These are in 

 the form of beads, streaks, or patches. They have an inde- 

 pendent motion, which is increased by the movements of the 

 eye. They are of greater specific gravity than the medium in 

 which they are found, and are supposed to be the remains of 

 the embryonic structure of the vitreous body. 



Under normal conditions the pupil appears as a black spot. 

 The reason for this is that the source of light and the retina 

 lie in conjugate foci, so that any light which escapes absorption 

 by the retinal pigment is reflected back whence it came. There- 

 fore, the eye of an observer who views it from another direction 

 will see no light coming from it. By means of an ophthal- 

 7noscope, however, a strong light is thrown into the fundus of the 



