THE SENSE OF TASTE 317 



the eye. This is known as presbyopia. Its remedy is the 

 same as that for hyperopia. 



Reaction to Light. Regarding the reaction of the pupil 

 to light, it is evident that this is mainly a reflex nervous phe- 

 nomenon, though direct light will cause the muscular tissue 

 of the iris to contract. The direct influence of the third 

 nerve on the action of the iris has been referred to under a 

 consideration of that nerve. Reflexly, the pupil is con- 

 tracted by light by the conveyance of an impression to the 

 brain through the optic fibers, a message is sent to the pro- 

 per center, and a stimulus is reflected through the third nerve 

 to the sphincter of the iris causing it to contract. When the 

 optic nerve is cut the circuit is broken, and movements of 

 the iris do not occur from the admission of light. Practic- 

 ally, then, when much or little light reaches the retina the pu- 

 pil contracts or dilates, as the case may be, in an effort to 

 keep the amount constant. 



Binocular Vision. It is evident that when a person looks 

 at an object two images are formed one on each retina 

 but they are combined in his consciousness and he sees but 

 one object. If one of the balls be thrown out of the proper 

 axis, by pressure, e. g., objects appear double. The same is 

 true in strabismus, at least until the person has grown ac- 

 customed to the defect. In normal vision the rays from 

 an object are formed on the fovea centralis of each eye that 

 is, upon corresponding points which are, for each, the centers 

 of distinct vision. 



4. The Sense of Taste. 



In order that gustatory sensation may be exercised it is 

 necessary (i) that there be specially endowed nerves and 

 nerve centers; (2) that the nerve terminals be excited by 

 sapid (tastable) materials; (3) that these substances be in 

 solution. It has already been seen that the special nerves of 

 taste are (a) the chorda tympani distributed to the anterior 



