1910.] The Tympanic Membrane. 127 



(King'sley, 1900, fig. 3), the tympanic cavity is seen to agree with that of the 

 mammals in arising immediately externally to the cochlea and aliove and 

 inside the cornn of the hyoid {cf., Kingsley, /. c, fig. 1, and Mead, 1909, fig. 4). 

 In the lizard (r/., Versluys, 1899, fig. 1) and in Ornithorhynchus (Weber, 

 1904, p. 145) the Eustachian tube is replaced by a wide communication 

 between the tympanic and the pharyngeal cavities. It therefore seems 

 reasonable to regard the tympanic cavity of the lizard as in the main 

 homologous with that of the mammal. 



The iijmpcmic membrane. 



From the considerations which follow it appears likely that the tympanic 

 membrane of mammals may be considered as homologous, at least in part, 

 with the tympanum of reptiles. The tympanic membrane in man (Cunning- 

 ham, 1902, pp. 706-707) consists of three layers: (1) the stratum cutaneum, 

 continuous with the external meatus; (2) the membrana propria, consisting 

 of two sets of fibres, radial and circular, which center around the handle of 

 the malleus; (3) the stratum mucosum, continuous with the general mucous 

 lining of the tympanic cavity. In the tympanum of rei)tiles the stratum 

 cutaneum and the stratum mucosmii are present but the membrana propria 

 is said to be lacking (Denker, 1901, p. 658). Versluys, however (1899, p. 

 359), in describing the tympanum of lizards speaks of a middle layer of 

 connective tissue provided with elastic fibres (and apparently homologous 

 with the membrana propria) as sometimes occurring. It is however gen- 

 erally vestigial, he says, and in this case the lizard tympanum would be 

 structurally comparable only with the dorsal segment of the mammalian 

 tympanum the "pars flaccida," where also the membrana propria is lacking. 



A further correspondence between the tympanum of reptiles and that of 

 mammals lies in the fact that the reptilian extracolumella and its supposed 

 homologue the mammalian manubrium mallei are in both cases inserted 

 between the inner and outer layers of the tympanum {of. Versluys, 1899, p. 

 360, Kingsley, 1900, p. 232 and Cunningham, 1902, p. 707). The reptilian 

 difl^ers from the mammalian tympanum in its location, since it is attached to 

 the posterior border of the quadrate and to the articular region of the mandi- 

 ble and therefore lies much above the three-layered portion of the membrane 

 in mammals. But in the Cynodont reptiles (Fig. 1, B, mb. iy.) the quadrate 

 is so much reduced that the location of the tym})anic membrane must have 

 corresponded more nearly with that of mammals. Here, as in so many 

 other characters, the Cynodonts may have bridged over the morphological 

 gap between reptiles and mammals. 



