CAVITY OF THE TYMPANUM. 453 



places in which the cartilage is deficient. The cerumen is 

 fluid at first, and gradually thickens. In some diseases of the 

 meatus it has the appearance of pus. The use of this secretion 

 is probably to exclude insects, for which it is well calculated 

 by its tenacity and bitterness ; and also to defend the delicate 

 surface upon which it is spread. 



The direction of the meatus is inwards and forwards : it is 

 also curved, with the convexity upwards ; but it is very easy, 

 by management of the external ear, to admit the rays of the sun 

 to the bottom of the meatus, and bring the membrana tympani 

 into view. 



Its length, in the adult, is about ten or twelve lines. It 

 is a little greater along the inferior part of the meatus, in conse- 

 quence of the oblique manner in which the canal is closed at its 

 internal extremity, by the membrana tympani. 

 The cerumen is composed, according to Vauquelin, of a fatty 

 oil, of a peculiar albuminous substance, and yellow coloring 

 matter. According to Rudolphi and Fourcroy, the bitter prin- 

 ciple of the cerumen is the same as that of the bile. 

 The external ear is a simple acoustic instrument, a sort of 

 trumpet, for the purpose of catching the sonorous vibrations, 

 and transmitting them to the membrane of the tympanum, to 

 which it is connected by the meatus auditorious externus. Its 

 removal, however, does not appear to impede, very seriously, 

 the function of hearing, as the rays still find their way down 

 the meatus. It is not found in purely aquatic animals, but in 

 those only which hear through the medium of the air. In 

 some birds, and some reptiles, there are, in front of the tym- 

 panum, two valves or lips, susceptible of opening and shutting, 

 like the eyelids. 



A vertical section of the meatus is of the oval figure. In the 

 foetus the bony part of this duct is not formed, and the meatus 

 consists entirely of cartilage. 



Of the Cavity of the Tympanum. 



The meatus auditorius externus is terminated abruptly by 

 the membrana tympani, which forms a septum that closes it 

 completely. 



