LECT. XIX. HEARING. 347 



extremely in different animals. In some it is reduced to 

 an apparatus of the greatest possible simplicity ; namely, to 

 a special nerve, whose peripheric extremity is expanded in 

 a liquid contained in a cavity of variable form, situated 

 sometimes in the osseous wall of the skull, sometimes hav- 

 ing membraniform envelopes. We shall give a particular 

 description of the human ear, because it is to this that we must 

 devote our especial attention, on account of its complication 

 and perfection. 



The exterior part of this organ, called the pinna or auri- 

 cle of the ear, is of a fibre-cartilaginous nature, very pliant 

 and elastic, and has the greatest portion of its surface free. 



It is, in a manner, an expansion of the external auditory 

 passage. The form of the pinna varies greatly in the higher 

 animals: thus, in man, although it presents a number of 

 folds, we may consider it as normally implanted in con- 

 nexion with the auditory tube. .In the horse, the ass, &c., 

 it consists of a species of cone, which arises from this tube. 

 In these animals the pinna is generally moveable ; whilst in 

 man its motions are very limited. 



The external auditory passage, or meatus auditorius exter- 

 nus, excavated in the temporal bone, terminates at a short 

 distance from the surface, where it is truncated obliquely 

 to its axis. A thin, and very elastic membrane, called the 

 membrane of the tympanum, closes the passage. The tym- 

 panum is a cavity, in great part osseous, which has five 

 openings: one formed by the extremity of the external 

 auditory passage, and closed by the membrane of the tym- 

 panum; two opposite to this, one situated above and called 

 the oval window (fenestra ovalis,) and the other the round 

 window (fenestra rotunda ;) both openings are closed by 

 membrane. Superiorly and posteriorly the tympanum com- 

 municates by a large irregular opening with the mastoid 

 cells. Lastly, in the lower part of the tympanum there 



