EXTERNAL AND MIDDLE PORTIONS OF THE EAR 767 



the reception of the head of the malleus. This articulation is effected in a plane 

 situated somewhat above the brim of the eardrum. This ossicle presents two 

 processes, the largest of which measures 4.5 mm. and the other 3.0 mm. in length. 

 To begin with the former extends downward, parallel to the manubrium of the 

 malleus, but suddenly turns inward to enter into articulation with the stapes. 

 The short process is fastened to the posterior wall of the tympanum by a thick 

 ligament which, however, gives rise to only a partial fixation of this bone. 



The stapes or stirrup bone, is only 2.5 mm. in length and weighs about 3 mgrs. 

 Its base is oval in shape and is fastened to the membrane of the fenestra ovalis by 

 means of a number of radial fibers of connective tissue. This foramen measures 

 3 mm. in length and 1.5 mm. in width. 



The Movements of the Ossicles. — It need scarcely be emphasized 

 that the function of the ear bones is to convert the vibrations of the 

 eardrum into vibrations of the membrane closing the fenestra ovalis. 

 This implies that the undulations in air are converted into oscillations 

 of the lymphatic fluid filling the spaces of the internal ear.^ The 

 latter then activates the constituents of the organ of Corti. In endeav- 

 oring to analyze the action of the ossicles it must be remembered that 

 the manubrium of the malleus is firmly anchored to the eardrum and 

 must, therefore, move in harmony with the latter. This fact may be 

 demonstrated in a very convincing manner by placing the umbo under 

 the ocular of a microscope, ^ inserted through a perforation in the 

 upper wall of the tympanum. When measured with the help of a 

 micrometer, these movements are seen to attain a maximal amplitude 

 of about 0.2 to 0.7 mm. 



Inasmuch as the neck of the malleus is fixed by the anterior and 

 posterior ligaments, the inward movement of its manubrium must give 

 rise to an outward deviation of its caput. This simple pendular 

 motion, however, cannot become excessive, because the malleus 

 executes at the same time a rotatory movement around its long axis. 

 The outward inclination of the caput of the malleus in turn enforces 

 a movement of the head of the incus in the same direction. At this 

 very moment the latter is turned as a whole around the axis formed by 

 its short process, while its long process is raised and is forced inward 

 against the stapes, pushing the latter more deeply into the foramen 

 ovale. The outward movement of the eardrum produces a movement 

 of these ossicles in the opposite direction. Helmholtz has compared 

 the malleus-incus articulation to the joints of a Breguet watch-key, 

 possessing a row of interlocking teeth which force the stem of the watch 

 in one direction, but prevent its revolution in the opposite direction. 



It will be seen, therefore, that this series of bones acts in the 

 manner of a bent lever, the fulcrum of which is placed at the tip of the 

 short process of the incus, while the power arm extends from here 

 through the tip of the manubrium, and the load arm, from here through 

 the tip of the long process of the incus. This arrangement is repre- 



1 Helmholtz, Pfluger's Archiv, i, 1869, 34. 



2 GoUtzer, Archiv fiir Ohrenheilkunde, i, 1864, 59, also see : Mach and Kessel, 

 Ber., Akad. der Wissensch., Wien, Ixix, 1874, 221. 



