178 



THE EAR. 



parent, oval membrane, separating the cavity of the tympanum from 

 the external ear. Its internal surface is slightly convex, and attached 

 to the handle of the malleus. It is directed obliquely downwards 

 and inwards, and it consists of three laminae, the middle of which is 

 strong and fibrous, and attached to the rim of the bone. The ex- 

 ternal surface is a continuation of the cuticle, and the internal of the 

 mucous membrane, lining the cavity. 



The internal wall of the tympanum has two orifices of communi- 

 cation with the internal ear ; the fenestra ovalis^ an oblong opening 

 leading to the vestibule, and the fenestra rotunda^ opening into the 

 cochlea; both of these are closed by membrane, which prevents the 

 escape of fluid contained in these inner chambers, and communi- 

 cates vibrations to it. The fenestra ova lis is also closed by the foot- 

 piece of the stapes. Between the fenestra is the promontory, an 

 elevation corresponding to the first turn of the cochlea, and furrowed 

 with two or three grooves for the nerves, which form the anastomosis 

 of Jacobson. The pyramid^ is a small conical projection behind 

 the fenestra ovalis, presenting a small orifice at its summit, through 

 which emerges the tendon of the stapedius muscle, which is con- 

 tained within, to reach the neck of the stapes. At the base of the 

 pyramid is an aperture through which the chorda tympani enters the 

 tympanum ; thence this nerve passes forwards between the handle of 

 the malleus and the long arm of the incus, and emerges through a 

 canal close to the Glaserian fissure. Above the pyramid an arched 

 prominence^ indicates the course of the aqueduct of Fallopius, close 

 to the tympanum, and behind this is the free communication with the 

 mastoid cells. At the upper part of the tympanum is the cochleari- 

 form process, which bounds the canal of the tensor of the tympani 

 muscle. Below this is the commencement of the Eustachian tuhe^ 

 which is a straight canal, about two inches long, at first bony, then car- 

 tilaginous, and opening by a trumpet-shaped orifice into the pharynx, 

 behind the inferior turbinated bone. 



The Bones of the tympanum are four in number ;^ the Qnalleus, or 



hammer, has a head^^ separated by 

 a neck from a handle^ which is im- 

 bedded in the drum of the ear. A 

 slight conical projection from the 

 neck, d^SXeA processus brevis, receives 

 the insertion of the tensor tympani 

 muscle. The processus gracilis is 

 long and thin, and extends from the 

 neck forwards and outwards to enter 

 the glenoid fissure. The back of the 

 head articulates with the incus. 



The Incus or anvil resembles a 

 molar tooth. Its body articulates with 

 the malleus. Its short cms*'' is di- 



