AUDITORY SENSATIONS 571 



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 would have a different periodicity, and the 

 membrane as a whole is 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 examina- 

 tion. On the inner side of the tympanic membrane is the tympanic 

 cavity, which is connected in front with the pharynx by means -oi the 

 Eustachian tube. This is opened by each movement of swallowing, 

 so that the pressure in the tympanum is kept equal to that of the out- 

 side air. When the Eustachian tube is blocked or diseased the air in 

 the tympanum is gradually absorbed and the patient becomes deaf on 

 that side. Stretching across the tympanum, from the membrana tym- 

 pani 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 manubrium, which is attached to the tym- 

 panic membrane, and the processus gracilis, 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 only of rotating 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, a 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 liga- 

 ment 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 outwards 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 when the handle of the 

 malleus moves inwards the greater part of the body of the incus 

 and of the head of the malleus move outwards together, while the 



