HEARING. 549 



reinforce the Sound. It is furnished with several muscles for the 

 purpose of changing its form ; but few individuals have the power 

 of using them. It is generally supposed that the habit has been 

 lost in most persons from the earliest infancy, on account of the 

 pressure of the coverings of the head. Be this as it may, some 

 persons preserve the power of controlling these muscles, arid I 

 have myself seen an individual in whom the motions arising from 

 their action were perfectly voluntary. 6 



The meatus auditorius is partly cartilaginous and partly bony. It 

 is lined by a bitter cerumen. f The external ear becomes tubular, 

 and thus continues to the osseous part, where it is terminated by 

 the membrane of the tympanum, the office of which is to receive im- 

 pressions from the agitations of the air, in order to transmit them 

 to the internal ear. e The membrane of the tympanum is of an 

 irregular conical form, something like a Chinese hat; its con- 

 cavity is on the outside, and its projecting point on the inside. 

 It is fixed to a bony rim which is called its frame. 



The cavity of the tympanum occupies the space between the 

 membrane and the labyrinth. It is an irregular cavity, nearly 

 hemispherical ; it is filled with air and communicates with the 

 back part of the mouth by means of a canal called the Eustachian 

 tube. 11 The side which is opposite the membrane presents an ob- 



e " V. J. Rhodius ad Scribon. Largum. p. 44. sq. 



J. Alb. Fabricius, De Hominibus ortu non differentibus. Opuscul. p. 441. 

 Ch. Collignon, Miscellaneous Works. Cambridge. 1786. 4to. p. 25. sq." 

 f " Consult J. Hay garth, Med. Obs. and Inquiries, vol. iv. p. 198. sq." 

 The cerumen consists, according to Vauquelin, of albumen, which, when 

 burnt, yields soda and phosphate of lime, a colouring matter, arid a very bitter 

 inspissated oil strongly resembling the peculiar matter of bile. Cicero explains 

 one use of the cerumen : " Provisum etiam, ut, si qua minima bestiola co- 

 naretur irrumpere, in sordibus aurium, tanquam in visco, inha^resceret." (J)e 

 Natura Deorum, 1. ii.) The same applies to particles of dust. Its extreme 

 bitterness, too, deters insects from advancing. 



g " See the distinguished Himly's acute comparison of the organs of hearing 

 and vision, Bibliotliekfiir Opthalmologie, vol. i. p. 6. sqq." 



h " Saunders, Anatomy of the Human Ear. Lond. 1806. fol. tab. i. ii." 

 " Comparative anatomy renders it most probable that the Eustachian tube 

 is subservient to the action of the membrana tympani. It is found in all red- 

 blooded animals which possess a membrana tympani, but is wanting in fishes 

 which are destitute of this membrane. The different opinions of the moderns 

 respecting its use may be found in Reil's Archiv fur die Physiol. t. ii. p. 18,, 

 iii. p. 165., iv. p. 105., viii. p. 67., ix. p. 320." 



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