750 SPECIAL SENSES. 



between the two layers of the fibrous structure of the membrana tympani 

 and occupies the upper half of its vertical diameter, extending from the pe- 

 riphery to the umbo. 



The membrana tympani, though thin and translucent, presents three dis- 

 tinct layers. Its outer layer is a very thin extension of the integument lining 

 the external meatus, presenting, however, neither papillae nor glands. The 

 inner layer is a delicate continuation of the mucous membrane lining the 

 tympanic cavity and is covered by tessellated epithelial cells. The fibrous 

 portion, or lamina propria, is formed of 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 mem- 

 brane, 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 addi- 

 tion there is 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 direction, and ^ to ^ of an inch (1-6 to 

 2*1 mm.) broad at its base. This appearance is regarded by physiologists as 

 very important, 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 upon which it is seen. 



Uses of the Membrana Tympani. It is unquestionable that the mem- 

 brana 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. As regards the mere acuteness of hearing, 

 aside from the pitch of sounds, the explanation of the action of the mem- 

 brane 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 vibrations are received by a membrane, under the conditions of the 

 membrana tympani, 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 well 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 



