190 



SENSE OF HEARING. 



of explanation. When the membrane is stretched in a direction paral- 

 lel to a sonorous surface, the oscillations, impressed upon it, are most 

 marked; and, accordingly, as sounds impinge upon the pavilion from 

 various quarters, the inequalities of surface always admit of some being 

 disposed in the most favourable way for the reception of vibrations. 

 It is true that the pavilion is not essential to audition ; the hearing not 

 suffering by its removal for more than a few days ; so that its physio- 

 logical influence is much more limited than might be conceived. It 

 probably contributes to a knowledge of the direction of sounds, and is 

 certainly calculated to protect the membrana tympani. 



The meatus auditorius externus conducts the sonorous vibrations 

 directly, and by reflection, as well as by its parietes, to the membrana 

 tympani. It is probable, too, that it is useful in protecting the mem- 

 brane from the direct action of air and extraneous bodies. This is, 

 perhaps, the cause of its tortuous character. If too much so, hearing 

 becomes impaired, the sonorous oscillations not being properly di- 

 rected towards the membrane. Baron Larrey has published cases of 

 deafness produced in this manner, which were removed by wearing an 

 artificial concha and meatus of the natural curvature made of gum 

 elastic. The down or hairs, at the entrance of the meatus, have been 

 regarded as protecting agents against the intrusion of extraneous bodies; 

 whilst the cerumen has been looked upon as a fit material for entrap- 

 ping insects in the slough formed by it, or for destroying them by its 

 poisonous influence. It is probable, however, that the most important 

 function of the cerumen is to keep the lining membrane of the meatus 

 in a physical condition adapted for the proper fulfilment of its func- 

 tions. 



Middle Ear. In the mode described, the vibrations of a sonorous 

 body attain the membrana tympani. An experiment by M. Savart 

 would seem to show, that the membrane is thrown into vibrations chiefly 

 by the air contained in the meatus. He made a small truncated cone 

 of pasteboard, and closed the narrow extremity by a tense mem- 

 brane, nearly as the membrana tympani closes the inner extremity of 

 the meatus auditorius ; and he found, that when sounds were produced 

 near the parietes of the cone, the membrane vibrated but little; whilst 

 if they were occasioned opposite the base of the cone, so that they 

 could be transmitted to the membrane by the air within the canal, the 

 vibrations were distinct, even at a distance of thirty yards and upwards. 



The membrane of the tympanum, then, receives and repeats the 

 sonorous vibrations. It has, however, been supposed to be pos- 

 sessed of other functions. M. Dumas 1 conceived it to be composed 

 of numerous cords, each corresponding to some particular tone. But 

 of this arrangement we have no evidence from observation or analogy. 

 By others, it has been supposed, and with probability, that the mem- 

 brane is capable of being rendered tense, or the contrary, by the bent 

 lever, formed by the chain of ossicles. They have farther presumed, 

 that this tension or relaxation is adapted to the sounds, which the mem- 

 brane has to transmit. The ancients believed, that the adaptation was 



1 Principes de Physiologic, 2de 6dit., Paris, 1806. 



