the Mode in which it administers to the Perception of Sound. 123 



may be described as consisting of the following layers : — (1) the 

 epidermis ; (2) the dermis ; (3) the fibrous layer, composed of 

 (a) the lamina of radiating fibres, (b) the lamina of circular 

 fibres ; (4) the mucous membrane*. 



It thus appears that the tympanum is a compound membrane 

 consisting of five layers which are mutually adherent, two of the 

 layers partaking of the character of fibrous membrane. 



Dr. Brennanf has furnished a Table of the principal organic 

 tissues in the order of their elasticity, which I give complete as 

 follows : — 



(1) Yellow fibrous tissue, (2) cartilage, (3) fibro-cartilage, 

 (4) skin, (5) cellular membrane, (6) muscle, (7) bone, (8) mu- 

 cous membrane, (9) serous membrane, (10) nervous matter, 

 (11) fibrous membrane. 



It thus appears that the tympanal membrane, instead of being 

 highly elastic, as it has been shown that it ought to be in order 

 to admit of the motion produced by waves of condensation being 

 transmitted through the tympanum, involves in its composition, 

 and has its elasticity measured by that of fibrous membrane, 

 which is the least elastic and the most unyielding of all the or- 

 ganic tissues, as to which Dr. Brennan observes that it " is re- 

 markable for its low degree of elasticity . ;; And that we may 

 be certain that the particular membrane of the tympanum is no 

 exception to the rule with regard to fibrous membrane in general, 

 we have the following testimony of Mr. Toynbee : — 



" Neither do the component fibres of the laminae appear to 

 evince more than an extremely slight degree of elasticity." 

 (Diseases of the Ear, p. 134 J.) 



Other arguments in favour of the position which I have been 

 seeking to establish will hereafter be adduced; and in particular 

 I shall endeavour to show that the auditory apparatus deprived 

 of the tympanal membrane, equally with the apparatus in its 

 normal state, is calculated to transmit waves of rarefaction and to 

 suppress waves of condensation ; but in the mean time I would 

 ask whether, if it had been the design of nature to secure such 

 transmission and suppression respectively in the perfect ear, any 

 construction of the tympanal membrane could have been devised 

 better calculated to accomplish those objects than that which ac- 

 tually occurs — the concavity of the membrane combined with its 

 flexibility ensuring the transmission of rarefied waves, whilst the 

 same concavity combined with inelasticity forbids the transmis- 

 sion of condensed waves. 



* Diseases of the Ear, with Supplement, by Hinton. London, 1868. 

 t Todd's ' Cyclopaedia of Anatomy and Physiology,' vol. ii. p. 60. 

 J " On n'y trouve point de fibres elastiques." — Traite d' Anatomie de- 

 scriptive, par Cruveilhier. Paris, 1868, vol. ii. p. 674. 



