152 SPECIAL SENSES. 



auditory meatns, which is closed internally by the membrana 

 tympani. 



2. The middle ear includes the cavity of the tympanum, 

 or drum, with its boundaries. The parts here to be described 

 are, the membrana tympani, the form of the tympanic cavity, 

 its openings, its lining membrane, and the small bones of the 

 ear, or ossicles, with their ligaments, muscles, and nerves. 

 The cavity of the tympanum communicates, by the Eu- 

 stachian tube, with the pharynx and also presents openings 

 into the mastoid cells. 



3. The internal ear contains the terminal filaments of the 

 auditory nerve. It includes the vestibule, the three semi- 

 circular canals, and the cochlea, which together form the 

 labyrinth. 



The pinna and the external meatus simply conduct the 

 waves of sound to the tympanum. The parts entering into 

 the structure of the middle ear are accessory, and are analo- 

 gous, in their functions, to the refracting media of the eye. 

 Structures contained in the labyrinth constitute the true 

 sensory organ ; and these bear the same relations to the au- 

 ditory apparatus as the retina to the eye. 



The External Ear. It is hardly necessary to our purpose 

 to describe very minutely the external ear. The pinna, or 

 auricle, is that portion projecting from the head, which first 

 receives the waves of sound. Beginning externally, we have 

 the helix, which is the outer ridge of the pinna. Just 

 within this, is a groove, called the fossa of the helix. This 

 fossa is bounded anteriorly by a prominent but shorter 

 ridge, called the antihelix ; and above the concha, between 

 the superior portion of the antihelix and the anterior portion 

 of the helix, is a shallow fossa, called the fossa of the anti- 

 helix. The deep fossa, immediately surrounding the open- 

 ing of the meatus, is called the concha. A small lobe pro- 

 jects posteriorly, covering the anterior portion of the concha, 

 and is called the tragus ; and the projection at the lower ex- 



