68 4 TEXT-BOOK OF PHYSIOLOGY. 



The quality of a sound depends on ihejorm of the vibration. It is this 

 feature which gives rise to those differences in sensations which permit one to 

 distinguish one instrument from another when both are emitting the same 

 note. The form of the sound-wave in any given instance is the resultant of a 

 combination of a fundamental vibration and certain secondary vibrations of 

 subdivisions of the vibrating body. These secondary vibrations give rise 

 to what is known as overtones. By their union with and modification of the 

 fundamental vibration there is produced a special form of vibration which 

 gives rise not to a simple but a composite sensation. It is for this reason that 

 the same note of the piano, the violin, and the human voice varies in quality. 



The Function of the Pinna and External Auditory Canal. In those 

 animals possessing movable ears the pinna plays an important part in the 

 collection of sound-waves. In man it is doubtful if it plays a part at all 

 necessary for hearing. Nevertheless an individual with defective hearing 

 may have the perception of sound increased by placing the pinna at an angle 

 of 90 degrees to the side of the head or by placing the hand behind it. The 

 external auditory canal transmits the sonorous vibrations to the tympanic 

 membrane. From the obliquity of this canal it has been supposed that the 

 vibrations, after passing the concha, undergo a series of reflections on their 

 way to the tympanic membrane, which, owing to its inclination, would be 

 struck by them in a much more effective manner. 



The Function of the Tympanic Membrane. The function of the 

 tympanic membrane is the reception of the atmospheric vibrations which 

 are transmitted to it. This it does by vibrating In unison with them. The 

 vibrations which the membrane exhibits correspond in amplitude, in fre- 

 quency, and in form to those of the atmosphere. That this membrane 

 actually reproduces all vibrations within the range of audibility has been 

 experimentally demonstrated. The membrane, not being fixed as far as its 

 tension is concerned, does not possess a fixed fundamental note, like a station- 

 ary fixed membrane, and is therefore just as well adapted for the reception of 

 one set of vibrations as another. This is made possible by variations in its 

 tension in accordance with the pitch or frequency of the atmospheric vibra- 

 tions. In the absence of vibration the membrane is in a condition of re- 

 laxation; with the advent of sound-waves possessing a gradual increase of 

 pitch, as in the ascent of the music scale, the tension of the membrane in- 

 creases until its maximum is reached at the upper limit of the range of 

 audibility. By this change in tension certain tones become perceptible and 

 distinct, while others become imperceptible and indistinct. 



The Function of the Tensor Tympani Muscle. The function of this 

 muscle is, as its name indicates, to change and to fix the tension of the tym- 

 panic membrane, so that it can most readily vibrate in unison with vibrations 

 of varying degrees of rapidity. The tendon of this muscle playing around 

 the processus cochleariformis is attached almost at a right angle to the handle 

 of the malleus. Hence as the muscle contracts it exerts its traction from the 

 process and draws the handle of the malleus inward, thus increasing the 

 convexity of the tympanic membrane and at the same time its tension. 

 With the relaxation of the muscle the handle of the malleus passes outward, 

 and the convexity and tension diminish. 



