48 THE REASON WHY I 



" Give every man thine ears, but few thy voice ; 

 Take each man's censure, but reserve thy judgment." SHAKSPKRE. 



power, we might have gone to sleep forgetting to have closed it, 

 thereby exposing the eye to considerable danger. But under the 

 involuntary arrangement the lid is made to fall over the eye as 

 drowsiness comes on, is kept there during sleep, and in the 

 morning, owing to the delicacy of its structure, it transmits a 

 sufficient quantity of light to the eye to arouse sensibility, and we 

 awake. 



144. Hoiv is the process of hearing conducted ? 



The folds of the outward ear conduct to its entrance, and into 

 an outer passage, which, after running inward a short distance, is 

 closed by a membrane called the drum. 

 Behind this membrane is an inner passage, 

 which terminates in the throat, which is 

 called the trumpet. Warm air from the 

 lungs therefore supplies the inner passage, 

 and the surrounding atmosphere fills the 

 outer passage. When the modification of 

 the atmosphere by which sounds are pro- 

 duced, flow into the outer passage, the drum 

 of the ear is put upon the stretch, more or less, according to the force 

 or loudness of the sound, as shown by the trick of calling loudly 

 under the pretence of whispering in the ear. Behind the drum, in 

 the hard bone, or the rock-like wall of the inner passage, are two 

 small openings termed mastoid, which communicate with the inner 

 cavities or chambers of the ear. There are two other openings 

 closed by a thin transparent membrane, and the chambers excavated, 

 as it were, in the rock-like bone, are, with the mastoid cells, 

 filled with fluid. Within these chambers (called the labyrinth and 

 semi-circular canals) the nerve of hearing spreads out between the 

 folds of a most delicate membrane, receiving the impressions 

 conveyed by the atmosphere or other conducting media, an<J 

 communicating them to the sense of hearing in the brain. 



145. Why do infants hear indistinctly? 



Because the bones of their ears are soft and cartilaginous ; and of 



