SPECIAL SENSES 



375 



If the eye has a defect of structure, as when the eyeball 

 is too short or too long, the picture is blurred because the 

 image is not formed exactly upon the retina. We say we 

 cannot see clearly. When the image is formed in front of 

 the retina, the defect is called near-sightedness or myopia. 

 When the image comes to a focus back of the retina, the 

 defect is called hypermetropia. In case of either of these 



Fig. 187. Semidiagrammatic section through the right ear. M, pinna; 

 G, meatus; T, tympanic membrane; P, tympanic cavity or middle 

 ear; R, Eustachian tube; r, fenestra rotunda; o, fenestra ovalis; V, 

 vestibule; B, a semicircular canal; S, cochlea; A, auditory nerve; 

 K, 1, 2, 3, 4, S, cartilages; Vt, passage of cochlea opening into vesti- 

 bule; Pt, passage of cochlea opening into tympanic cavity; ai, 1, 1', 

 ampullae of the semi-circular canals; c, organ of Corti. (After Martin.) 



troubles glasses should be worn to overcome the defects, 

 otherwise the strain upon the eye, in its effort to focus sharply, 

 will be so great as to affect the nervous system seriously and 

 cause headaches. 



Astigmatism is another common defect of the eye due to 

 irregular curvature of either the surface of the cornea or 



