(502 PHYSIOLOGY 



essential that it should be devoid of any periodicity, i.e. a tendency to 

 vibrate at a certain frequency. If such periodicity were present the ear 

 would pick out and magnify, to the exclusion of the other overtones, 

 some particular overtone present in the compound tones reaching the ear. 

 The perfect aperiodicity of the tympanic membrane is secured by its 

 structure and attachments. The membrane is composed of a thin 

 layer of fibrous tissue covered externally with skin and internally by the 

 mucous membrane of the tympanum. To its inner surface along its whole 

 length is attached the handle of the malleus, the first of the auditory ossicles. 

 This attachment of an elastic membrane to a mass of bone would itself tend 

 to damp any vibrations of the membrane. By the attachment of the 

 tendon of the tensor tympani muscle to the inner surface of the handle of the 

 malleus, the middle of the membrane is drawn inwards, so that it forms a cone 

 whose walls are convex outwardly. The membrane is built up of circular 

 and radial fibres, the circular being best marked towards the periphery. 

 By the dragging inwards of its central part it follows that the tension of its 

 constituent fibres varies from point to point so that each bit of the membrane 

 has a different periodicity, and the membrane as a whole be aperiodic. 



By exposing the tympanum from above it is possible with a micro- 

 scope to observe the actual movements of the handle of the malleus when 

 sound waves fall on the tympanic membrane. The maximum movements at 

 the apex of the cone may be taken as about '04 mm., but sounds are easily 

 audible which would produce movements of the tympanic membrane quite 

 imperceptible under this method of examination. 



THE OSSICLES. Stretching across the tympanum, from the membrana 

 tympani to the outer wall of the internal ear, is a chain of ossicles, which 

 are named respectively the malleus, the incus, and the stapes. These ossicles 

 are articulated together, so that a movement inwards of the malleus causes a 

 movement inwards of the base of the stapes. The malleus, or hammer bone, 

 consists of a thickened head, from which two processes run, viz. the ttmnn- 

 brium, which is attached to the tympanic membrane, and the processes 

 yracilis, by which it is anchored to the walls of the tympanic cavity. By 

 means of three ligaments it is so fixed that it is capable of rotating only 

 around a horizontal axis, which passes through the anterior ligament, the 

 head of the malleus, the body of the incus, and the short process of the incus. 

 When the manubrium is pushed inwards, the part of the malleus above this 

 axis must move outwards. The incus, sometimes known as the anvil bone, 

 is articulated with both the stapes and the malleus, and a ligament passes 

 from its short process to the posterior wall of the tympanic cavity. The 

 posterior surface of the rounded head of the malleus fits into the saddle- 

 shaped cavity on the anterior surface of the incus, while the tip of the long 

 process of the incus is articulated with the stapes. Movement inwards or 

 out \\,m Is of the head of the malleus causes rotation of the incus round an 

 axis which passes from the tip of the short process through its body. Thus 

 \\hcn the handle of the mallei^ moves inwards the greater part of the bod v of 

 the incus and of the head of the malleus moves outwards together, while the 



