260 ANIMAL PHYSIOLOGY. 



tympani tense, and the incus, partaking of the movement of 

 the malleus, pushes the stapes in at the fenestra ovalis, it is 

 plain that the whole contents of the internal ear are sub- 

 jected to pressure, and that the secondary membrana tym- 

 pani is likewise made tense. Thus a harmony is maintained 

 between the condition of the primary membrana tympani 

 and the internal ear. 



But it might happen that the nervous structures of the 

 ear might require protection from violent noise, as the retina 

 requires protection from excessive light, and gets it by excit- 

 ing a reflex action which contracts the pupil; for this reason 

 there is what may be described as a safety-valve arrangement 

 connected with the stapes. To the neck of the stapes a tendon 

 is attached, which passes back through a foramen in the 

 posterior wall of the tympanum, and, when the bone is 

 broken open, is seen to be continued into a muscle called the 

 stapedius muscle. This muscle, when it contracts, pulls the 

 stapes into an oblique position in the fenestra rotunda, and 

 interferes with the pressure of the incus, thus relieving the 

 inner ear from a state of tension. Judging from the anatomy 

 of the parts, we may conclude that this is a correct view of 

 their function, and fchat the stapedius muscle is stimulated 

 to contract by a reflex action, of which the auditory is the 

 incident or sensory nerve. 



193. The internal ear is imbedded in osseous tissue; and the 

 cavity which it occupies, the osseous labyrinth, can be well 

 studied in a macerated temporal bone. The petrous portion 

 of the temporal bone is a long three-sided pyramid of hard 

 consistence, with its base turned towards the tympanum; 

 and in its posterior wall, which looks into the cranium, there 

 is a large foramen, the meatus auditorius internus, directed 

 outwards. When this meatus is looked into, it is seen to 

 terminate very soon in a perforated plate, which occupies its 

 lower part, and leaves an opening into a canal above it. This 

 canal, termed aquceductus Fallopii, gives passage to the portio 

 dura of the seventh nerve, otherwise called the facial, and 

 has nothing to do with the organ of hearing; but the per- 

 forated or cribriform plate transmits the portio mollis or audi- 

 tory nerve to the internal ear; the perforations in its fore 

 and hinder moiety leading respectively into the anterior and 



