254 OUR BODIES AND HOW WE LIVE 



The hammer bone is fastened by its long handle to the 

 drum membrane. The round head of the hammer bone 

 fits into the anvil bone. Next to the anvil is the stirrup 

 bone, which fits into the little oval window in the opposite 

 wall of the chamber or drum. 



348. The Eustachian Tube. In the floor of the tym- 

 panic cavity is the opening of a passage called the Eusta- 

 chian tube. This tube is about an inch and a half long, and 

 leads into the back part of the throat. It allows air from 



the throat to enter the drum, and serves 

 to keep the air on both sides of it at 

 equal pressure (Fig. 69). 



During a severe cold in the head, or 

 a sore throat, the lining of the tube may 

 be inflamed and swollen. This gives 

 FIG. 157. The Bones a stuffed feeling in the ears, and the 

 of the Ear. hearing may be slightly impaired. As 



i, malleus, or hammer; tne cold paSSCS off, this peculiar feeling 

 2, incus, or anvil; 3, 



stapes, or stirrup. m the ears usually disappears. 



Experiment 76. To produce vibration of the tympanic membrane 

 and the little ear bones. Shut the mouth and pinch the nose tightly. 

 Try to force air through the nose. The air dilates the Eustachian 

 tube and is forced into the ear drum. The distinct crackle, or click- 

 ing sound, is due to the movement of the ear bones and the tympanic 

 membrane. 



349. The Inner Ear. The inner ear is a bony case filled 

 with liquid in which float the delicate ends of the nerve of 

 hearing. It consists of three distinct portions, the vesti- 

 bule, the semicircular canals, and the cochlea, or snail's shell. 

 It is enough for us to remember that these are winding 

 channels and spiral tubes hollowed out in the solid bone. 

 The whole system of passages is known as the labyrinth. 



