406 SPECIAL PHYSIOLOGY. 



regard it as also a tensor, but others as a laxator, of this membrane. 

 But these two muscles must also influence the condition of the mem- 

 brane which, with the base of the stapes, closes the foramen ovale. By 

 Fick, the tensor tympani, acting indirectly on the stapes, is said to 

 press inwards that bone ; if so, it must tighten the membrane of the 

 foramen ovale, and so increase the pressure, through the contents of 

 the labyrinth, on the membrane of the fenestra rotunda of the cochlea. 

 Concerning the action of the stapedius on this membrane, there is still, 

 however, considerable doubt : some suppose that it must aid the tensor 

 tympani in tightening, not only the membrana tympani, but also that 

 of the foramen ovale ; whilst others have conjectured, that it not 

 only relaxes the membrane of the drum, but also draws outwards that 

 of the oval foramen, and so diminishes the pressure, through the con- 

 tents of the labyrinth, on the membrane of the fenestra rotunda. 



In all the Vertebrate, and also in the aquatic non- Vertebrate animals, 

 the sonorous undulations traverse a fluid medium, before they impinge 

 on the extremities of the auditory nerve. In Man, and in the air- 

 breathing Vertebrata, the undulations propagated from the tympanum, 

 pass through the perilymph of the labyrinth, partly to the membranous 

 utricle, saccule, and semicircular canals, so as to reach the nervous 

 filaments contained in their interior, and partly along the scala vesti- 

 buli of the cochlea, pulsating over its denticulate zone, rods of Corti, 

 and radiating nerves, up to its very summit, and thence, down the 

 scala. tympani, to the membrane of the fenestra rotunda. The exact 

 use of the fenestra rotunda, and of the membrane closing it, is not well 

 understood. As already mentioned, it has even been supposed by 

 some, to receive vibrations directly from the air in the tympanum. 

 But if, in hearing, the sonorous vibrations pass through the ossicles to 

 the labyrinth, this memb.rane of the fenestra rotunda may either act 

 as a spring, protecting the structures of the cochlea from too strong or 

 sharp undulations, or it may prevent the return of those undulations 

 backwards, along the scalae of the cochlea. The weakness of the tym- 

 panic muscles, and the elasticity of the membranes which close in the 

 labyrinth, are opposed to the idea, often entertained, of the occurrence 

 of any great variation in the degree of tension of the fluid contents of 

 the labyrinth. The changes above described are mainly effected 

 through the increased, or diminished, pressure of the base of the stapes 

 at the fenestra ovalis. 



On the supposition that the tensor tympani and stapedius muscles 

 tighten both the membrana tympani and the membranes of the fora- 

 men ovale and fenestra rotunda, and necessarily also the joints of the 

 chain of ossicles between them, they must serve to protect the auditory 

 nerve from too powerful vibrations, and, at the same time, render the 

 auditory apparatus better adapted for the reception of high tones, and 

 less fitted for the reception of low notes. These offices are probably 

 regulated by reflex nervous action, and so may be compared with the 

 functions of the iris, in regard to the regulation by it of the quantity 

 of light which is admitted into the eye to act on the retina. But, on 

 the supposition that the tensor tympani tightens both the membrane 

 of the drum and the two membranes of the labyrinth, whilst the stape- 



