THE MIDDLE EAR. 



1493 



The cavity of the tympanum is subdivided into three parts : ( i ) the atrium or 

 tympanic cavity proper; (2) the caviim epitympanicum, the upper part of the space 

 which overhes the atrium ; and (3) the antrum^ which leads into the mastoid cells. 



The atrium (Fig-. 1251) resembles in shape a short cylinder with concave 

 ends, the outer end being formed by the tympanic membrane and its bony margin, 

 whilst the inner end is formed by the outer wall of the labyrinth. 



The cavum epitympanicum or attic occupies the space between the atrium 

 and the roof and constitutes approximately one-third (about 5 mm. ) of the supero- 

 inferior diameter. It contains the head of the malleus and the body of the incus 

 (Fig. 1252). It extends considerably over the external auditory canal and is 

 bounded laterally by a wedge-shaped portion of the temporal bone, called the scutum. 



The antrum tympanicum is an irregularly pyramidal space communicating 

 with the upper back part of the tympanum by a triangular orifice. Its dimensions 

 vary, but its average length is about 12 mm., its height 8.5 mm., and its width 6.7 

 mm. It is larger in the infant than in the adult, and its lumen is frequently lessened 

 by bands of mucous membrane which stretch across it and thus encroach upon the 

 space. Its roof is formed by the tegmen tympani, sometimes called the tegmcn 

 antri in this location. Its external wall is formed by the squamous portion of the 



Ftg. 1252. 



Superior ligament of incus 

 Superior ligament of malleus 

 Head of malleus. 



Chorda tympani nerve 



Tensor tympani 

 Processus cochleariformis 



Eustachian tube 



■Epitympanic space 

 Incus 



Orbicular process, for stapes 

 Handle of malleus 



— Tympanic membrane 



Inner aspect of outer wall of right tympanic cavity, showing incus and malleus and tympanic 



membrane in position. X 2}^. 



temporal bone, and on its internal one is seen the outer wall of the horizontal semicir- 

 cular canal. The thin mucous membrane of the antrum is closely united with the 

 periosteum and possesses a layer of low nonciliated squamous epithelium. 



The walls of the tympanic cavity present many irregularities and depres- 

 sions and the boundaries are not sharply defined. As the direction of the supero- 

 inferior axis of the cavity is not perpendicular but oblique, it follows that the outer 

 wall, composed of the tympanic membrane and its bony margin, is, accurately 

 speaking, the infero-lateral wall, whilst that formed by the labyrinth is the dorso- 

 mesial wall. For convenience of description, however, there may be recognized 

 with advantage an external and an internal, a superior and an inferior, and an 

 anterior and a posterior zvall. 



The outer wall (paries membranacea) of the tympanic cavity proper (the 

 atrium) is formed by the drum-head ahd the margin of bone into which it is inserted, 

 whilst the outer wall of the epitympanic space is formed by the scutum. In the infant 

 the bony external auditory canal consists of a ring of bone, the annulus tympani- 

 cus. This ring, incomplete at its upper anterior part at the notch of Rivinus, 

 possesses a well-marked groove, the sulcus tympanicus, for the reception of the 

 tympanic membrane. At the notch of Rivinus, the tympanic membrane is attached 

 to the bony margo tympanicus and the external lateral ligament of the malleus, and 

 is continuous with the skin lining the bony auditory canal. 



