760 PHYSIOLOGY. 



cortex of the parietal and central convolutions. Some fibers from 

 the nucleus of Deiters and of Bechterew go by the posterior longi- 

 tudinal bundle to the nuclei of the motor nerves of the eye. (See 

 equilibratory center of Mills.) 



The cochlear nerve is the nerve concerned in hearing. 



The vestibular nerve is the nerve concerned in equilibration. It 

 does not have anything to do with hearing. 



Membrana Tympani. The membrana tympani is an elastic, 

 very vascular membrane, which protects the delicate organs of the 

 middle ear against the action of cold coming in from the external 

 ear. It is also specially endowed with a specific sensibility for the 

 contact of special agents, as the scratchings of an insect on its surface 

 cause a peculiar auditory sensation. The membrana tympani is of 

 variable size, according to the species of animal, and is adapted to 

 receive low and high sounds. It is of circular form, and attached 

 by its borders upon a bony circle, the tympanic circle. Its direction 

 is peculiar. It cuts obliquely the axis of the external auditory 

 meatus and this obliquity is favorable to the impact of sound-waves. 

 It is depressed and becomes prominent in the middle, having the 

 arrangement of a depressed cone. Under the shock of sound-waves 

 the membrana tympani vibrates for all sounds in the range of per- 

 ceptible sounds. Its vibration can be measured by a water mano- 

 meter inserted into the external auditory canal. 



Accommodation of Membrana Tympani. Since the membrana 

 t} 7 mpani vibrates in unison with all the external sounds which strike 

 it, it is inferrable that there is a means capable of regulating the 

 tension of this membrane. The shape of the tympanic membrane 

 is peculiarly adapted for transforming weak movements of wide 

 amplitude into strong ones of wide compass. For it is not simply a 

 depressed cone, but the radii are slightly curved with the convexity 

 outward, a shape mainly caused by the elastic fibers maintaining a 

 tension on its inner surface, these being most numerous toward the 

 circumference. The principal regulator of the tension is the tensor 

 iympani. The membrane of the tympanum has no definite funda- 

 mental tone; it vibrates indifferently to every sound. The mem- 

 brana tympani is tense for high sounds and relaxed for low sounds, 

 but these changes in tension are chiefly for the intensity of sound 

 rather than their height, so as to offer a resistance to the shock of 

 sound-waves and obviate the effect of this shock upon the deep and 

 delicate structure of the ear. 



The adherence of the membrana tympani to the handle of the 



