THE ORGANS OF SENSE 



309 



the glare from the paper tends to fatigue the eyes, and 

 reading during convalescence from illness is especially apt 

 to leave the eyes in a weakened condition. 



The ear, or organ of hearing, is composed of three parts, 

 the external, the middle and the internal ear. The external 

 ears of most of the mammals can be turned so as to catch 

 the sound, or else flattened against the head for protection, 

 but in ourselves the ear muscles are mere rudiments 

 capable of producing only a slight amount of movement, 



SC 



FIG. 222. Section through the ear; a, anvil; h, hammer; s, stirrup; 

 co, cochlea; eo, external opening; Et, Eustachian tube; tb, temporal 

 bone;' tc, tympanic cavity; tm, tympanic membrane; v, vestibule. 



or none at all. In fact the aural appendages of man are 

 almost devoid of function as they are poorly adapted for 

 catching sound and represent but the vestiges of a struc- 

 ture once valuable to the animal ancestors of man. The 

 passage leading toward the inner parts of the ear is 

 furnished with numerous glands that secrete a waxy 

 substance which serves to protect the ear from dust and 

 insects. The inner end of the passage is closed by the 

 tympanic, or drum membrane. On the inner side of this 



