214 THE HEAD AND NECK 



aditus that infection reaches the tympanic antrum from the 

 tympanic cavity. 



The Auditory Ossicles. The handle of the malleus is very intimately 

 connected to the membrana tympani. Waves of sound impinging on the 

 membrane thrust the malleus medially, and the movement is conveyed through 

 it to the incus and stapes. Medial movements of the stapes set up a wave 

 in the perilymph of the scala vestibuli, and this wave passes up the cochlea 

 to the scala tympani. In the cochlea it stimulates the spiral organ of Corti, 

 and it is dissipated below, where it impinges on the membrane which closes 

 the fenestra of the cochlea (fenestra rotunda). 



The Tensor Tympani occupies a small canal immediately above the 

 auditory tube. Its tendon is inserted into the upper part of the handle of 

 the malleus, and its contractions draw the bone medially, thus rendering the 

 tympanic membrane tense. It is supplied by a branch from the otic ganglion, 

 which lies on the lateral wall of the nasal pharynx, and obtains its motor fibres 

 from the mandibular division of the trigeminal. The Stapedius emerges 

 from a small opening in the posterior wall of the tympanic cavity, and is 

 inserted into the neck of the stapes. It is supplied by the facial nerve just 

 before it reaches the stylo-mastoid foramen. In paralysis of this nerve, 

 proximal to the origin of the branch to the stapedius, the condition of hyper- 

 acousis occurs as a result of paralysis of the muscle. 



The auditory ossicles and the tendons of their muscles are all covered by 

 the mucous membrane, which extends backwards from the naso-pharynx 

 along the auditory tube, through the tympanic cavity and aditus into the 

 tympanic antrum and its furthest ramifications. 



The Tympanic (Mastoid) Antrum, which is present at 

 birth, is relatively larger in the child than in the adult. It lies 

 behind and slightly above the tympanic cavity, with which it 

 communicates thiough the aditus. Its lateral wall, formed 

 by the squamous portion of the temporal, is only one-sixteenth 

 of an inch thick at birth, but by the ninth year it has increased 

 to nearly half an inch. The floor and medial wall, formed by 

 the petrous portion of the temporal, separate the antrum from 

 the posterior cranial fossa and the descending portion of the 

 transverse (lateral) sinus, which forms a deep groove on the in- 

 ternal surface of the mastoid portion of the temporal bone. 

 The aditus is placed above the level of the floor, and consequently 

 can only act as an overflow when the antrum contains fluid. 

 The roof is formed by a backward continuation of the tegmen 

 tympani. 



As the mastoid process develops at the end of the second 

 year its diploe is gradually replaced by air-cells, which are 

 derived from the antrum and maintain their connection 

 with it, either directly or indirectly through neighbouring cells. 

 Occasionally the process is entirely hollowed out by air-cells, the 

 pneumatic type, or it may be practically solid, the sclerosed 

 type. Various intermediate forms occur. The cells which lie 



