190 HAND-BOOK OF PHYSIOLOGY. 



brana tympani. Miiller has confirmed this by experiments with small 

 membranes arranged so as to imitate the membrana tympani; and it may 

 be confirmed also by observations on one's self. 



The pharyngeal orifice of the Eustachian tube is usually shut; during 

 swallowing, however, it is opened; this may be shown as follows: If the 

 nose and mouth be closed and the cheeks blown out, a sense of pressure 

 is produced in both ears the moment we swallow; this is due, doubtless, 

 to the bulging out of the tympanic membrane by the compressed air 

 which, at that moment, enters the Eustachian tube. 



Similarly the tympanic membrane may be pressed in by rarefying the 

 air in the tympanum. This can be readily accomplished by closing the 

 mouth and nose, and making an inspiratory effort and at the same time 

 swallowing (Valsalva). In both cases the sense of hearing is temporarily 

 dulled; proving that equality of pressure on both sides of the tympanic 

 membrane is necessary for its full efficiency. 



Functions of Eustachian Tube. The principal office of the 

 Eustachian tube, in M tiller's opinion, has relation to the prevention of 

 these effects of increased tension of the membrana tympani. Its exist- 

 ence and openness will provide for the maintenance of the equilibrium 

 between the air within the tympanum and the external air, so as to pre- 

 vent the inordinate tension of the membrana tympani which would be 

 produced by too great or too little pressure on either side. While dis- 

 charging this office, however, it will serve to render sounds clearer, as 

 (Henle suggests) the apertures in violins do; to supply the tympanum 

 with air; and to be an outlet for mucus. If the Eustachian tube were 

 permanently open, the sound of one's own voice would probably be greatly 

 intensified, a condition which would of course interfere with the percep- 

 tion of other sounds. At any rate, it is certain that sonorous vibrations 

 can be propagated up the Eustachian tube to the tympanum by means of 

 a tube inserted into the pharyngeal orifice of the Eustachian tube. 



Action of Tensor Tympani. The influence of the tensor tympani 

 muscle in modifying hearing may also be probably explained in connec- 

 tion 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, just as the iris and orbicularis palpebrarum muscle are by 

 a very intense light, then it is manifest 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 per- 

 sons of irritable nervous system, a sudden contraction of many muscles. 



"The ossicula of aquatic mammalia are very bulky and relatively large, 

 especially in the true seals and the sirenia (Manatee and Dugong). In 

 the cetacea the stapes is generally ankylosed to the fenestra ovalis, the 

 malleus is always ankylosed to the tympanic bone, yet the membrana tym- 

 pani is well formed, and there is a manubrium, often ill-developed, but 

 always attached to the membrane by a long process. In the Otarise or Sea- 



