vm 



THE BONY LABYRINTH 379 



other structures developed from the original sac. is lodged 

 in chambers of the petrous part of the temporal bone. 



In the fresh state, this collection of chambers m the 

 petrous bone is perfectly closed ; but, in the dry skull, 

 there are (as we have seen, p. 370) two wide openings, 

 termed fenestrse, or windows, on its outer wall : i.e., 

 on the side nearest the outside of the skull. Of these 

 fenestra, one, termed ovalis (the oval window), is 

 situated in the wall of the vestibular cavity ; the other, 

 rotunda (the round window), behind and below this, 

 is, as we have seen, the open end of the scala tymjxmi 

 at the base of the spiral of the cochlea. In the fresh 

 state, each of these windows or fenestrse is closed by a 

 fibrous membrane, continuous with the periosteum of the 



bone. 



The fenestra rotunda is closed by membrane only ; but 

 fastened to the centre of the membrane of the fenestra 

 ovalis, so as to leave only a narrow margin, is an oval 

 plate of bone, the foot of the stapes (p. 370). 



(ix) The Transmission of Sound \Vaves to the 

 Inner Ear.— The manner in which the complex apparatus 

 now described intermediates between the physical agent, 

 which is the primary cause of the sensation of sound, and 

 the nervous expansion, the aflfection of which alone can 

 excite that sensation, must next be considered. 



All bodies which produce sound are in a state of vibra- 

 tion, and they communicate the vibrations of their own 

 substance to the air with which they are in contact and 

 thus throw that air into waves, just as a stick waved 

 backwards and forwards in water throws the water into 



waves. 



The aerial waves, produced by the vibrations of sono- 

 rous bodies, in part enter the external auditory passage, 

 and in part strike upon the concha of the external ear and 

 the outer surface of the head. It may be that some of the 

 latter impulses are transmitted through the solid struc- 

 ture of the skuU to the organ of hearing ; but before they 



