358 THE SPECIAL SENSES. 



of these movements under normal conditions is very minute. 

 That of the base of the stirrup is estimated at about 0.04 

 mm., while the amplitude at the tip of the manubrium, though 

 relatively much larger, is still less than a millimeter (0.2 to 

 0.7 mm.). The minute but relatively intense movements of the 

 stapes set into vibration the perilymph in the internal ear, and 

 through these movements the sensory nerve cells in the cochlea 

 are stimulated, and nerve impulses are thereby aroused in the 

 fibers of the cochlear nerve. Ankylosis of the ear bones impedes 

 their movements and impairs the delicacy of hearing, and if the an- 

 kylosis affects the base of the stapes at its insertion into the fenestra 

 ovalis practically complete deafness ensues. The articulation of 

 the head of the malleus with the body of the incus is a peculiar 

 saddle-shaped joint, which, according to the description given by 

 Helmholtz, acts like a cogged or ratchet movement. When the 

 tympanic membrane moves in and the head of the malleus, there- 

 fore, moves outward, the joint locks, so that the incus follows the 

 malleus. If, however, from any unusual cause the tympanic mem- 

 brane is moved outward from its resting position, as may result, for 

 instance, from a marked fall in air pressure, then the malleus-incus 

 joint unlocks and the incus fails to follow completely the movement 

 of the malleus, thereby protecting the structures in the internal ear. 

 Muscles of the Middle Ear. Two small muscles are present in 

 the middle ear : the tensor tympani and the stapedius. The former 

 arises in a groove just above the Eustachian tube and its long tendon 

 is inserted into the neck of the malleus just below the axis of rotation. 

 The muscle is innervated by a branch of the fifth nerve. It is 

 obvious that when this muscle contracts it must pull the tympanic 

 membrane inward and put it under greater tension. The stapedius 

 muscle arises from the inner wall of the tympanic cavity and its 

 tendon is inserted into the neck of the stapes. This muscle is 

 innervated through a branch of the facial. When it contracts it 

 tends to pull the stapes laterally, and thus probably places the mem- 

 brane attached to its base under greater tension. The functions 

 fulfilled by these muscles have been the subject of much controversy. 

 According to a view first proposed by Johannes Muller, they act as 

 a protective mechanism to the membranes of the middle ear. By 

 increasing the tension of the membranes they limit the amplitude of 

 their vibrations and thus protect the membranes from injury or 

 possible rupture in the case of the violent movements resulting from 

 loud, explosive noises. Or possibly by their reflex contraction 

 they protect us from intense, disagreeable noises, by limiting the 

 responsiveness of the vibrating membranes. A more probable view, 

 however, and one supported to some extent by experimental evi- 

 dence, was suggested by Mach. According to this observer, the 



