Nervous System and Sense Organs 221 



glands which keep the canal moist as well as collect dust and other 

 foreign particles. Hairs within this canal also help to remove foreign 

 objects. 



Between the tympanic membrane and the inner ear is the air- 

 filled middle ear. This chamber is actually a heritage from the embry- 

 onic gill pouches. As such, it retains its connection with the pharynx 

 through the Eustachian tube. By means of this tube, the air pressure 

 on both sides of the eardrum is kept equal. The opening of this tube 

 from the pharynx is closed except during swallowing, yawning, or blow- 

 ing of the nose. Unfortunately this connection with the mouth pre- 

 sents certain disadvantages, for infections of the throat can easily pass 

 into the Eustachian tube and then into the middle ear. If one ascends 

 a mountain rapidly or flies in an airplane, some discomfiture is ex- 

 perienced in the middle ear due to the trapping of air. The discom- 

 fiture is due to the expansion of the trapped air and can be relieved 

 by forcibly opening the Eustachian tube. During some respiratory 

 infections, the tube may be plugged with mucus and give a sensation 

 of partial deafness due to the une(fual pressure against the tympanic 

 membrane. The inequality of pressure in this case is due to the fact 

 that the air trapped in the middle ear is gradually absorbed by the 

 blood. 



The air-borne vibrations are transmitted to the inner ear across 

 the middle ear by a series of three small bones : the malleus or hammer, 

 the incus or anvil, and the stapes or stirrup. The malleus is fixed to 

 the eardrum and connects with the incus. The long process of the incus 

 connects to the stapes which is in turn fixed to the membrane of the 

 oval window of the inner ear. 



The inner ear consists of two main regions : the cochlea which is 

 concerned with hearing, and the semicircular canals which are con- 

 cerned with balance. This portion of the ear is imbedded within the 

 petrous portion of the temporal bone in the winding cavities known as 

 the bony labyrinth. In one part of these cavities is the cochlea, in the 

 other the semicircular canals. Between these two structures is a con- 

 necting portion, the vestibule, which consists of two small sacs, the 

 sacculus and the utriculus. The sacculus, in part, gives rise to the 

 cochlea. 



The cochlea is a membranous structure lying in the bony labyrinth. 

 Due to its spiral shape it is often named the snail shell. If the cochlea 

 were uncurled it would be seen to consist of three canals : the vestibu- 

 lar, the tympanic, and the cochlear. The inner tympanic canal is, in 



