32 MOVEMENTS OF THE SMALL BONES OF THE EAR. 



field of vision does not result from a coalescence of two perspective 

 views of it depicted on the retinae, but that the single idea thus excited, is 

 the product entirely of a mental operation, by which a single form is 

 created from the two images presented to the retinae. The distinction is 

 one obviously unimportant, and has probably arisen from these observers 

 having under- estimated the amount of mental influence admitted by 

 Professor Wheatstone to be concerned in the true visual perception of a 

 solid body.* 



OF THE SENSE OF HEARING.! 



Movements of the small bones of the ear. In an account of the movements 

 undergone by the small bones of the ear, and their utility to the sense of 

 hearing, Ed. Weber J offers an explanation of the rounded appearance pre- 

 sented by the external surface of the membrane of the fenestra rotunda 

 when the membrana tympani is pushed inwards after death, and of its con- 

 cave appearance when the membrana tympani is drawn outwards. He 

 observes that the articulation between the head of the malleus and the body 

 of the incus is such that the former bone cannot be moved alone by the 

 membrana tympani, but that both move together as one bone. He states 

 that their axis of movement is the line drawn from the slender process of 

 the malleus to the short process of the incus ; on these two processes ad- 

 herent to the wall of t the tympanum the bones turn, as on a pivot. Thus 

 it happens that when the membrana tympani is pushed inwards, the stapes 

 is pressed within the fenestra ovalis by the long process of the incus ; when, 

 on the contrary, the membrana tympani is drawn outwards, the stapes is 

 carried out from the fenestra ovalis. The stapes could not exercise these 

 movements completely if the cavity of the labyrinth was bounded entirely 

 by firm and unyielding walls, for the fluid within it is almost incompres- 

 sible. But by the pressure of the fenestra rotunda this difficulty is avoided. 

 The fluid which fills the vestibule communicates with that in the cochlea, 

 particularly with that of the scali vestibuli, which again freely com- 

 municates with that of the scala tympani ; so that when the membrane of 

 the fenestra ovalis is pushed inwards towards the vestibule, the consequent 

 pressure on the contained fluid will be communicated to the membrane of 

 the fenestra rotunda, which will be pushed outwards. In this way the 

 movements of the membrana tympani produce indirectly the flux and 

 reflux of the fluid of the labyrinth from the fenestra ovalis to the fenestra 

 rotunda, by percussion, and by the yielding of the lamina spiralis of the 

 cochlea. 



* See Muller's Physiology, pp. 1200 and 1206. 



t Section ii. p. 1215, Mullet's Physiology. 



t Archiv. d'Anat. Physiol. Janvier, 1846, p. 16. 



