HEARING. 



329 



pursuits. Myopia is rare among those who cannot read or 

 who live mainly out of doors. It is not so apt to lead to 

 permanent injury of the eye as hypermetropia, but the 

 effort to see distinctly any but near objects is apt to pro- 

 duce headaches and other symptoms of nervous exhaus- 

 tion. If the myopia becomes gradually worse, the eyes 

 should be rested for several months. 



Hearing. The auditory organ (Fig. 95) consists of three 

 portions, known respectively as the external ear, the middle 



FIG. 95. Semi-diagrammatic section through the right ear. M, concha. (?, 

 external auditory meatus. 2'. tympanic or drum membrane. P, Tympanum, 

 o, oval foramen, r. round foramen. Extending from 7'to o is seen the chain of 

 tympanic bones. R, Eustachian tube. V, B, S, bony labyrinth: F", vestibule; 

 B, semicircular canal ; S, cochlea. 6, 1, I', membranous semicircular canal 

 and vestibule. A, auditory nerve dividing into branches for vestibule, semi- 

 circular canal, and cochea. 



ear or tympanum, and the internal ear or labyrinth ; of 

 these the latter is the essential one, containing the ends of 

 the auditory nerve-fibres. 



What is apt to result from short sight? 

 Of what does the auditory organ consist? 



