Page:Observations on Man 1834.djvu/175

 obliquus act, greater when these are relaxed, and the internus acts. The degree of sensibility both in the membrane itself, and in the whole organ, is probably greater when the tension is greater.

The vibrations of the air seem to shake off the mucus which lodges upon the membrana tympani in the new-born child, just as the action of light fits the cornea, and aqueous humour, for vision.

The eustachian tube serves to supply the cavity of the tympanum with air, to carry off the vapour which exhales into it, and perhaps to increase the effects of sounds by the tremors in its cartilaginous extremity, and particularly those of the person’s own voice.

The stapes, its muscle, and the foramen ovale, seem designed to convey the vibrations of the membrana tympani to the auditory nerve, in a precise manner. But the vibrations excited in its æther may be much more frequent than those of the membrana tympani, as has already been observed.

There does not appear to be any method for conveying air into the cavities of the vestibulum cochlea, and semi-circular canals, nor any necessity or use for it there. The great hardness of the bony part of the organ of hearing may make it more easily susceptible of vibrations. Agreeably to which, it may be observed, that we can hear imperfectly, though the ears, nose, and mouth, be all stopped, and consequently all access to the membrana tympani by the vibrations of the air denied. The vibrations are here excited probably in the cartilages of the auricle and meatus, and in the bones of the skull, and thus communicated to the cochlea, and semi-circular canals. It seems to agree with this, that some partially deaf persons can hear best, when driven in a coach over stones or gravel; i.e. when all the parts of their bodies, and particularly those of the bones, are put into a vibrating motion.

The bony part of the organ of hearing seems to come to its full size early in life. Is not the final cause of this, that one part may, on account of its size, be most disposed to vibrate with one tone, another with another? And does not this hold particularly in respect of the lamina spiralis? For thus the same tone would be affixed and associated to the same nerves, so as to affect them peculiarly, from childhood or youth to old age.



we may consider, first, the different strength of sounds. Now, as this arises from the different condensation of the air in the pulses, so the effect which it has upon the membrana tympani, the membrane of the foramen ovale, and the nerves of