USES OF DIFFERENT PARTS OF THE AUDITORY APPARATUS. 837 



seen, with proper illumination, a well-marked, triangular cone of light, with its apex at 

 the end of the handle of the malleus, spreading out in a downward and forward direc- 

 tion, and from T \ to T V of an inch broad at its base. This appearance is regarded by 

 pathologists as very important, as indicating a normal condition of the membrane. It is 

 undoubtedly due to reflection of light, depending upon three factors, indicated by Roosa 

 as follows : " First, the inclination of the membrana tympani to the auditory canal 

 second, the traction of the malleus, which renders it concave at the centre ; third, its 

 polish or brilliancy." With this explanation, it is not admitted that the light spot is due 

 to a peculiar structure of that portion of the membrane upon which it is seen. 



Uses of the Membrana, Tympani. It is unquestionable that the membrana tympani is 

 very important in audition. In cases of disease in which the membrane is thickened, 

 perforated, or destroyed, the acuteness of hearing is always more or less affected. That 

 this is in great part due to the absence of a vibrating surface for the reception of waves 

 of sound, is shown by the relief which is experienced by those patients who can tolerate 

 the presence of an artificial membrane of rubber, when this is introduced. As regards 

 the mere acuteness of hearing, aside from -the pitch of sounds, the explanation of the 

 action of the membrane is very simple. Sonorous vibrations are not readily transmitted 

 through the atmosphere to solid bodies, like the bones of the ear ; and when they are 

 thus transmitted they lose considerably in intensity. When, however, the aerial vibra- 

 tions are received by a delicate membrane, under the conditions of the membrana tym- 

 pani, they are transmitted with very little loss of intensity ; and, if this membrane be 

 connected with solid bodies, like the bones of the middle ear, the vibrations are readily 

 conveyed to the sensory portions of the auditory apparatus. The parts composing the 

 middle ear are thus admirably adapted to the transmission of sonorous waves to the 

 auditory nerves. The membrane of the tympanum is delicate in structure, stretched to 

 the proper degree of tension, and vibrates under the influence of the waves of sound. 

 Attached to this membrane, is the chain of bones, which conducts its vibrations, like 

 the bridge of a violin, to the liquid of the labyrinth. The membrane is fixed at its 

 periphery and has air upon both sides, so that it is under the most favorable conditions 

 for vibration. 



A study of the mechanism of the ossicles and muscles of the middle ear shows that 

 the membrana tympani is subject to certain physiological variations in tension, due to 

 contraction of the tensor tympani. It is also evident that this membrane may be drawn 

 in and rendered tense by exhausting or rarefying the air in the drum. If the mouth and 

 nose be closed and we attempt to breathe forcibly by expanding the chest, the external 

 pressure tightens the membrane. In this condition, the ear is rendered insensible to 

 grave sounds, but high-pitched sounds appear to be more intense. If the tension be re- 

 lieved, as may be done by an act of swallowing, the grave sounds are heard with normal 

 distinctness. This experiment, tried at a concert, produces the curious effect of abolish- 

 ing a great number of the lowest tones, while the shrill sounds are heard very acutely. 

 The same phenomena are observed when the external pressure is increased by descent in 

 a diving-bell. 



Undoubted cases of voluntary contraction of the tensor tympani have been observed 

 by otologists ; and in these, by bringing this muscle into action, the limit of the perception 

 of high tones is greatly increased. In two instances of this kind, recorded by Dr. Blake, 

 of Boston, the ordinary limit of perception was found to be three thousand single vibra- 

 tions ; and, by contraction of the muscle, this was increased to five thousand single vibra- 

 tions. 



The membrana tympani undoubtedly vibrates by influence, when it is brought in 

 accord with a given note. In other words, this membrane obeys the laws of consonance 

 and vibrates strongly by the influence of sounds in unison or in harmony with its funda- 

 mental tone. The laws of vibrations by influence have already been fully discussed ; 



