736 SPECIAL SENSES 



two layers. The outer layer consists of fibres radiating from the handle 

 of the malleus to the periphery. These are best seen near the centre. 

 The inner layer is composed of circular fibres, which are most abundant 

 near the periphery and diminish in number toward the centre. 



The color of the membrana tympani, when it is examined with an 

 aural speculum by daylight, is peculiar, and it is rather difficult to 

 describe, as it varies in the normal ear in different individuals. Politzer 

 described the membrane, examined in this way, as translucent, and of a 

 color which " most nearly approaches a neutral gray, mingled with a 

 weaker tint of violet and light yellowish brown." This color is modified, 

 in certain portions of the membrane, by the chorda tympani and the 

 bones of the ear, which produce some opacity. The entire membrane 

 in health has a soft lustre. In addition there is seen, with proper illu- 

 mination, 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 for- 

 ward direction, and -^ to ^ of an inch ( 1.6 to 2.1 millimeters) broad at 

 its base. This appearance is regarded by physiologists as indicating a 

 normal condition of the membrane. It is undoubtedly due to reflection 

 of light and not to a peculiar structure of that portion of the membrane 

 on 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 acute- 

 ness of hearing is 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 experienced by patients who can tolerate the 

 presence of an artificial membrane of rubber. 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 consid- 

 erably in intensity. When, however, the aerial vibrations are received by 

 a membrane, under the conditions of the membrana tympani, they are 

 transmitted with little loss of intensity ; and if this membrane is con- 

 nected 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 well adapted to the transmis- 

 sion 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 angular chain of bones, which con- 

 ducts its vibrations, like the bridge of a violin, to the liquid of the 



