THE SPECIAL SENSES 309 



Fig. 70, A, d). From the peculiar arrangement 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. 



96. The circular membrane, already mentioned as closing the 

 auditory canal, is the partition which separates the middle 

 from the external ear, and is called the membrana 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 



FIG. 71. SHOWING THE INTERNAL MECHANISM or THE EAR (GREATLY ENLARGED") 



is incorrect, since a drum is not a membrane, but is the hollow 

 space across which the membrane is stretched. This mem- 

 branous drum-head is very tense and elastic, and so thin as to 

 be almost transparent; its margin is fastened into a circular 

 groove in the adjacent bone. Each wave of sound that touches 

 this delicate membrane causes it to vibrate, and it, in turn, 

 excites movements in the parts beyond. 



97. Within the tympanum is arranged a chain of remark- 

 able "little bones," or ossicles. They are chiefly three in 



96. What is the membrana tympani ? Describe it. 



71. What are the ossicles ? Their number and names ? Their arrangement ? 



