THE SPECIAL SENSES. 219 



and is slightly curved; so that the inner end is ordinarily 

 concealed from view. The pouch of the skin which lines 

 this passage is smooth and thin, especially at the lower 

 end, where it covers the membrane just mentioned. 



93. As in the case of the nostrils, a number of small, 

 stiff hairs garnish the margin of the auditory canal, and 

 guard it, to some extent, against the entrance of insects 

 and other foreign objects. The skin, too, covering its outer 

 half, is furnished with a belt of little glands which secrete 

 a yellow, viscid, and bitter substance, called " ear-wax," 

 which is especially obnoxious to small insects. As the 

 outer layer of this wax-like material loses its useful pro- 

 perties, it becomes dry, and falls out of the ear in the form 

 of minute, thin scales, a fresh supply being furnished from 

 the little glands beneath. In its form, the auditory canal 

 resembles the tube of an ear-trumpet, and serves to convey 

 the waves of sound to the middle portion of the ear. 



94. The Middle Ear, or Tympanum. The middle 

 ear is a small cavity, or chamber, of irregular shape, about 

 one-fourth of an inch across from side to side, and half an 

 inch long (see Fig. 53, A, d). From the peculiar arrange- 

 ment of its various parts it has very properly been called 

 the tympanum, or the "drum of the ear." The middle 

 ear, like the external canal, contains air. 



95. The circular membrane, already mentioned as clos- 

 ing the auditory canal, is the partition which separates 

 the middle from the external ear, and is called the mem- 

 brana tympani (c), and may be considered as the outer head 

 of the drum of the ear. It is sometimes itself spoken of as 

 the " drum," but this is incorrect ; since a drum is not a 

 membrane, but is the hollow space across which the mem- 

 brane is stretched. This membranous drum-head is very 

 tense and elastic, and so thin as to be almost transparent; 



93. How i* it guarded and protected ? Ear-wax ? 



94. What is the middle ear 'i Why called tympanum ? 



95. What, is the membraua tympani ? Describe: it. 



