THE SENSES. 689 



The influence of the tensor tympani muscle in modifying hearing 

 may also be probably explained in connection with the regulation of the 

 tension of the membrana tympani. If, through reflex nervous action, 

 it can be excited to contraction by a very loud sound, then it is mani- 

 fest that a very intense sound would, through the action of this muscle, 

 induce a deafening or muffling of the ears. In favor of this supposition 

 we have the fact that a loud sound excites, by reflection, nervous action, 

 winking of the eyelids, and, in persons of irritable nervous system, a 

 sudden contraction of many muscles. 



The exact influence of the stapedius muscle in hearing is unknown. 

 It acts upon the stapes in such a manner as to make it rest obliquely in 

 the fenestra ovalis, depressing that side of it on which it acts, and ele- 

 vating the other side to the same extent. It prevents too great a move- 

 ment of the bone. 



The fluid of the labyrinth is the most general and constant of the 

 acoustic provisions of the labyrinth. In all forms of organs of hearing, 

 the sonorous vibrations affect the auditory nerve through the medium 

 of liquid the most convenient medium, on many accounts, for such a 

 purpose. 



The otoliths in the labyrinth would reinforce the sonorous vibrations 

 by their resonance, even if they did not actually touch the membranes 

 upon which the nerves are expanded ; but, inasmuch as these bodies lie 

 in contact with the membranous parts of the labyrinth, and the vestibu- 

 ^ar nerve-fibres are imbedded in them, they communicate to these mem- 

 branes and the nerves, vibratory impulses of greater intensity than the 

 fluid of the labyrinth can impart. This appears to be their office. So- 

 norous undulations in water are not perceived by the hand itself immersed 

 'n the water, but are felt distinctly through the medium of a rod held 

 n the hand. The fine hair-like prolongations from the epithelial cells 

 if the ampullae have, probably, the same function. 



The function of the semicircular canals in the co-ordination of 

 movements necessary to the maintenance of the equilibrium of the body 

 las already been indicated. 



The cochlea seems to be constructed for the spreading out of the 

 lerve-fibres over a wide extent of surface, upon a solid lamina which 

 iommunicates with the solid walls of the labyrinth and cranium, at the 

 iime time that it is in contact with the fluid of the labyrinth, and 

 vhich, besides exposing the nerve-fibres to the influence of sonorous 

 mdulations, by two media, is itself insulated by fluid on either side. 



The connection of the lamina spiralis with the solid walls of the 

 ,byrinth, adapts the cochlea for the perception of the sonorous undula- 

 ions propagated by the solid parts of the head and the walls of the laby- 

 inth. The membranous labyrinth of the vestibule and semicircular 

 44 



