ORGAN OF HEARING IN MAMMALIA. 



239 



communicates with the labyrinth by the foramen ovale, b, and fora- 

 men rotundum, c ; with the exterior of the cranium by the foramen 



auditorium externum : 

 but all these apertures 

 are closed by membrane 

 in the recent state. The 

 other communications are 

 with the breathing pass- 

 age, back of the nose, or 

 pharynx, by the eusta- 

 chian tube, fig. 167, a f 

 b, c, whereby air is con- 

 veyed into the tympa- 

 num, and thence passes 

 into the mastoid cells. 

 On the petrosal wall of 

 the tympanic cavity is 

 specified the ( promon- 

 tory,' «, between the openings, b, c, the pyramid, d, the eminence 

 of the ' fallopian aqueduct,' e, and the groove, f, for the internal 

 ligament of the malleus. 



The movements of the membrane closing the foramen ovale, b, 



Termination of cochlear nerve, more highly magn. 

 (A.nat. size). xcvr\ 



184 



185 



The inner wall of the tympanum, xcvii" 



EkjTUUnoBa] and tympanic bone with the 

 membrane. Human foetus, xcvii". 



are brought into relation with those of the membrane closing the 

 outer auditory opening by the chain of ossicles called ( otosteals.' 

 The ' membrana tympani ' is fixed in a groove of a bony frame 

 which is so far ossified as to form an incomplete ring, at the third 

 month of human foetal life ; at the sixth month it begins to coa- 

 lesce with the squamosal, fig. 185, and then to grow outward, 

 forming the wall of the bottom of the auditory meatus, fig. 188, g, 

 the lower part of which is the last to be completed. The drum, 

 fig. 186, consists of a ( proper membrane,' with an inner layer 



