734 SPECIAL SENSES 



USES OF DIFFERENT PARTS OF THE MIDDLE EAR 



The uses of the pavilion and of the external auditory meatus are 

 sufficiently plain. The pavilion serves to collect the waves of sound, 

 and probably it inclines them toward the external meatus as they come 

 from various directions. Although this action is simple, it has some 

 importance, and the various curves of the concavity of the pavilion tend 

 more or less to concentrate sonorous vibrations. Such has long been 

 the opinion of physiologists, and this seems to be carried out by experi- 

 ments in which the concavities of the external ear have been obliterated 

 by wax. There probably is no resonance or vibration of much impor- 

 tance until the waves of sound strike the membrana tympani. The same 

 remarks may be made in regard to the external auditory meatus. It is 

 not known precisely how the obliquity and the curves of this canal 

 affect the waves of sound, but it is probable that the deviation from a 

 straight course protects, to a certain extent, the tympanic membrane 

 from impressions that might otherwise be too violent. 



Structure of the Membrana Tympani. The general arrangement of 

 the membrana tympani has already been described in connection with 

 the topographical anatomy of the auditory apparatus. The membrane 

 is elastic, about the thickness of ordinary gold-beater's skin, and is 

 subject to various degrees of tension by the action of the muscles of the 

 middle ear and under different conditions of atmospheric pressure within 

 and without the tympanic cavity. Its form is nearly circular ; and it has a 

 diameter in the adult, according to Sappey, of a little more than f of an 

 inch (10 to ii millimeters) vertically and about \ of an inch (10 milli- 

 meters) antero-posteriorly. The excess of the vertical over the hori- 

 zontal diameter is about -$ of an inch (0.5 millimeter). 



The periphery of the tympanic membrane is received into a little 

 ring of bone, which may be separated by maceration in early life but is 

 consolidated with the adjacent bony structures in the adult. This bony 

 ring is incomplete at its superior portion, but aside from this, it resem- 

 bles the groove which receives the crystal of a watch. At the periphery 

 of the membrane, is a ring of condensed fibrous tissue which is received 

 into the bony ring. This ring also presents a break at its superior 

 portion. 



The concavity of the membrana tympani presents outward, and it 



comprehended without a general knowledge of the physics of sound and of some of the laws of 

 harmony ; for not only is there a perception of single notes by the auditory apparatus, but the 

 most elaborate combinations of sounds in harmony are appreciated together and at one and the 

 same instant, as will be seen in studying the action and uses of different parts concerned in audi- 

 tion. Many of the laws of musical combinations are directly applicable to the physiology of 

 hearing. 



