THE NERVOUS SYSTEM 197 



the ligament. The lens then thickens and increases its convexity 

 until checked by the tension of the capsule. The response of the 

 lens to changes in tension of the capsule and ligaments is due to 

 its elasticity. Loss of elasticity is a natural accompaniment of 

 old age and results in a loss of accommodation. 



Acuity of vision also depends upon the amount of light enter- 

 ing the eye. This is controlled by the size of the pupil, which is 

 contracted in a bright light and enlarged in a dim light. The 

 "color" of the eye is determined by the amount and distribution 

 of the pigment in the iris. Dark-brown and black eyes have 

 pigment in the outer surface, through the stroma, and in the 

 inner surface of the iris; light-brown, gray, and green eyes have 

 less pigment in the outer surface; while blue eyes have pigment 

 only in the inner surface of the iris. The pigment is the same 

 substance in all cases. 



Eye Muscles. — By means of six extrinsic eye muscles, the eye- 

 ball of vertebrates, including that of Man, can be moved about 

 in the orbit in all directions through a considerable arc. The 

 internal and external recti muscles move the eye from side to side ; 

 the superior and inferior recti muscles move it up and down; 

 and the superior and inferior oblique muscles rotate it. These 

 muscles have their origin in the wall of the orbit and are inserted 

 on the sclerotic surface of the eyeball. They are innervated by 

 cranial nerves III, IV, and VI. In the frog there are two addi- 

 tional eye muscles, the retractor bulbi, which pulls the eyeball 

 into the orbit, and the levator bulbi, which raises the eye. 



Visual Purple. — The change undergone by the rods and 

 cones through which the nervous impulse is set up is generally 

 believed to be photochemical in nature. By this is meant that 

 light generates chemical changes in the rods or cones, similar in 

 a general way to the effect of light on a photographic plate or 

 film. This view is based on the fact that the outer segments of 

 the rods contain a reddish pigment, called visual purple or 

 rhodopsin that is bleached by light. This pigment is distinct 

 from the pigment of the pigmented layer of the retina, though 

 this layer seems to play some part in restoring the visual purple 

 in the rods following its destruction by light. It would seem, 

 however, that visual purple is not essential to vision since it is 

 absent in the cones, which means that it is absent from the 

 fovea centralis, the region of greatest visual acuity. It has been 



