THE SENSE ORGANS 



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covered with a tympanum or drum membrane, and possibly the 

 antennae are sensitive to sound vibrations as well. Ear organs 

 may be located on legs, abdomen, antennae, and head in various 

 animals. 



Structure of the Human Ear. The vertebrate ear is a wonder- 

 fully complicated organ, consisting of an external ear which opens 



Auditory 

 nerve 



FIG. 134. 



Basilar membrane 



Eustachian tube 



Semi-diagrammatic section through the right ear. 

 (After Martin.) From Kellogg. 



into an auditory canal embedded in the skull. This canal is closed 

 at its inner end by the tympanic membrane, which separates it 

 from the middle ear. 



The middle ear connects with the throat by way of the eusta- 

 chian tube which serves to equalize the air pressure on both sides 

 of the drum and thus prevents breakage, while permitting free 

 vibration. Across the middle ear extends a chain of tiny bones 

 which connects the tympanic membrane with a somewhat similar 

 membrane in the wall of the inner ear. 



The internal ear consists of two general parts. The cochlea is a 

 cavity in the skull shaped like a snail shell, filled with a liquid and 



