1088 



SPECIAL SENSE ORGANS 



The Tympanic Cavity 



The tympanic cavity [cavum tympani], as has been stated, is an air-space, 

 Hned with mucous membrane, situated between the external and the internal ear. 

 It is of irregular outline, but, roughly, it is a slit-like cavity, lying in an obhque 

 antero-posterior plane. Its transverse diameter measures only from 2-4 mm., 

 while the vertical and antero-posterior diameters measure about 15 mm. (fig. 834). 



It is narrowest at the centre, and wider superiorly than inferiorly. The bony 

 walls have already been partly described with the temporal bone, and hence the 

 description given here will refer to the appearance found in the fresh, or un- 

 macerated condition. 



It will be noticed (see fig. 829) that the floor of the space is on very much 

 the same horizontal plane as the floor of the external meatus, and the lower 

 margin of the tympanic membrane. The roof, on the other hand, lies at a much 



Fig. 832. — Lateral Surface of the Left Membrana Tympani. (Enlarged from life.) 

 Pars flaccida or Shrapnell's membrane Posterior plica 



Anterior plica 



Malleolar prominence caused by 

 lateral process of malleus 



Umbo, corresponds to tip of 

 manubrium of malleus 



Cone of light 



Long process of incus 

 Malleolar stria 

 "Pars tensa of tympanic membrane 



higher level than the upper margin of that membrane. Hence the cavity may 

 be divided into two regions, a lower part, corresponding in extent to the tym- 

 panic membrane, and an upper, above the upper border of the membrane, known 

 as the epitympanic recess. This division forms a definite chamber, and con- 

 tains the head of the malleus and the body and short process of the incus. 

 It is on the posterior part of this chamber that the communication with the 

 tympanic antrum is found (fig. 835). 



As the shape of the tympanum is irregular, its walls are not everywhere 

 clearly marked off from one another, but there may be recognized (figs. 829 

 and 835) a roof, or tegmental wall, a floor, or jugular wall, a medial or labyrin- 

 thine wall and a lateral or membranous wall, an anterior or carotid, and a pos- 

 terior or mastoid boundary or wall. 



The roof, or tegmental wall, is formed by a portion of the tegmen tympani, a thin plate 

 of bone which is continued backward to form the roof of the tj'^mpanic antrum. This plate is 

 formed by the petrous part of the temporal bone, and at its lateral margin is the petro-squamous 

 suture, wherfi a slight deficiency in the roof may occur. 



The floor, or jugular wall is very narrow transversely, and is in intimate relation to the 

 internal jugular vein (fig. 831). As shown in fig. 833, the surface is frequently very irregular 

 from stalactite-like projections between which are the tympanic celluhc (air cells), while near 

 the back there is occasionally a marked projection corresponding externally to the root of the 

 styloid process. 



The posterior or mastoid wall presents at its lower part, many additional tympanic cellulac, 

 and higher up, an elevation, the pyramidal eminence, on whose apex is an aperture transmitting 

 the tendon of tlie stapedius muscle. The fleshy belly of that muscle is contained in a cavity 

 in the interior of the bony pyramid of the posterior wall. Lateral to this is an aperture, the 

 aperlura tympanica canaliculac chords, through which the chorda tympani nerve enters the tym- 

 panum, covered by a reflexion of the mucous membrane. Between this opening and the pyra- 

 mid is a slight elevation; and ul)Ove it in a fossa, termed the sinus posterior. Above this again 

 is a recess, where the posterior ligament of the incus is attached, known as the fossa incudis. 

 This portion of the posterior wall forms the boundary of the epitympanic recess. Here the 

 cavity of the tympanum is continued with that of the antrum tympanicum, or mastoid antrum, 



