USES OF DIFFERENT PARTS OF THE AUDITORY APPARATUS. 835 



When arranged in this way, the membrane can be made to sound its fundamental note 

 by influence. In addition, the membrane, like a string, will divide itself so as to sound the 

 harmonics of the fundamental, and it will likewise be thrown into vibration by the 5th 

 3d, etc., of its fundamental tone, thus obeying the laws of vibrations of strings, though 

 the harmonic sounds are produced with greater difficulty. 



Uses of Different Parts of the Auditory Apparatus. 



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

 apparent. 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 undoubtedly has a certain degree of importance, and the various curves of 

 the concavity of the pavilion tend more or less to concentrate the sonorous vibrations. 

 Such has long been the opinion of physiologists, and this seems to be carried out by 

 experiments in which the concavities of the external ear have been obliterated by wax. 

 There is, probably, no resonance or vibration of much importance until the waves of 

 sound strike the membrana tympani. The same remarks may be made with regard to 

 the external auditory meatus. We do not know precisely how the obliquity and the 

 curves of this canal affect the waves of sound, but we may suppose that the deviation 

 from a straight course protects, to a certain degree, the tympanic membrane from im- 

 pressions 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. This structure, which is of great importance in the physiology of 

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

 subject to various degrees of tension, from the action of the muscles of the middle ear 

 and different conditions of atmospheric pressure within and without the cavity of the 

 tympanum. Its form is nearly circular. From a number of accurate measurements of 

 its diameter in the adult, by Sappey, we may assume that its ring measures a little more 

 than f of an inch vertically and about f of an inch antero-posteriorly. The excess of the 

 vertical over the horizontal diameter is about -^ of an inch. Notwithstanding the asser- 

 tion of some of the older anatomists, that the tympanic membrane presents one or two 

 small perforations, it is now almost universally regarded as forming a complete division, 

 without openings, between the external meatus and the middle ear ; or, if any openings 

 exist, they are exceedingly minute. 



The periphery of the tympanic membrane is received into a little ring of bone, which 

 may be separated by maceration in early life, but which is consolidated with the adja- 

 cent bony structures in the adult. This bony ring is incomplete at its superior portion, 

 but, aside from this, it resembles 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 may be increased or 

 diminished by the action of the muscles of the middle ear. The point of greatest con- 

 cavity, where the extremity of the handle of the malleus is attached, is called the umbo. 

 Upon the inner surface of the membrane are two pouches, or pockets. One is formed by 

 a small, irregular, triangular fold, situated at the upper part of its posterior half and con- 

 sisting of a process of the fibrous layer. This, which is called the posterior pocket, is open 

 below and extends from the posterior upper border of the membrane to the handle of 

 the malleus, which it assists in holding in position. " After it has been divided, the bone 

 is much more movable than before." (Troltsch.) The anterior pocket is lower and shorter 

 than the posterior. It is formed by a small bony process turned toward the neck of the 

 malleus, by the mucous membrane, by the bony process of the malleus, by its anterior 



