316 THE SENSES 



that nerve. Reflexly, the pupil is contracted by light by the 

 conveyance of an impression to the brain through the optic fibers, 

 a message is sent to the proper 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. 

 Practically, then, when much or little light reaches the retina 

 the pupil 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 accustomed 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 neces- 

 sary (i) that there be specially endowed nerves and nerve centers; 

 (2) that the nerve terminals be excited by sapid (tastable) ma- 

 terials; (3) that these substances be in solution. It has already 

 been seen that the special nerves of taste are (a) the chorda tym- 

 pani distributed to the anterior two-thirds of the tongue, and (&) 

 the glosso-pharyngeal to the posterior third of that organ. It 

 is probable that only the dorsum of the tongue, the lateral parts 

 of the soft palate, the uvula and the upper pharynx are concerned 

 in gustation. On the tongue are found special papillae, (i) the 

 circumvallate, large and few in number, near the base of the 

 organ, and (2) the fungiform, about 200 in number, over the 

 remaining area. The circumvallate and some of the fungiform 



