THE SPECIAL SENSES 



127 



colors. The slowest vibrations are red. The most rapid 

 violet. A mixture of all the colors of the spectrum makes 

 white light, or light without color, a result of the min- 

 gling of the various rates of vibration of the different 

 colors. In order that the eye may serve its purpose with- 

 out constantly turning the head, the eyeball is provided 

 with ocular muscles which can move the ball in all direc- 

 tions; and, that the amount of light may be neither too 



Fig. 29. Section through the anterior portion of the eye: C, the cornea; 

 I, the iris (note the circular muscular fibers cut across at the margin) ; L, 

 the lens; Ci, the ciliary process; S, the suspensory ligament; Scl, the scle- 

 rotic or outer protective coat of the eye. (From a preparation by P. M. Spur- 

 ney.) (Pearce-Macleod, Fundamentals of Human Physiology.) 



great nor too little, provision must be made for regulat- 

 ing the size of the opening through which light is admit- 

 ted to the interior of the eyeball. This is accomplished 

 by putting a curtain, with a hole in the middle, in the 

 fore part of the eyeball, and providing it with muscular 

 fibers which can increase or dimmish the size of the hole. 



