Fig. 23. Vertical Section through the Left Temporal Bone in the 

 plane of the Axis of the Petrous Portion. 



Frozen Section. Mucous Membrane of Tympanic Cavity, Antrum and Mastoid 



Cells in red. The lower part of tlie Tympanic Cavity has l)een carried aivay 



in the section so f/iat the External Auditory Canal is exposed. 



Fig. 24. Horizontal Section through the Left Temporal Bone. 



Section through a macerated bone. Tlie axis of the Mucous Membrane lining 

 the Eustachian Tube and the Mastoid Antrum is indicated b\ a red line. 

 The axis througli the External and Internal Axiditory Meatus, "Sensory Axis", 

 is indicated by a yellow line. Tlie Cartilage of the Temporo-Mandibular Articu- 

 lation is coloured blue. 



The middle ear is surrounded by numerous air-cells which aire lined by 

 a continuation of the same mucous membrane and communicate directh' or indi- 

 rectly with the Tympanic Cavity. Pus spreads readily from the Tympanic Cavit}^ 

 to these Accessory Cells. Fig. 23 shews how the cells lie below the plane of the 

 opening into the r3'mpanic Cavity and consequentl}' drainage is very efficient. 

 The size of these air-cells varies not only on the same side but on either side in 

 the same individual. These cells maj- extend far into the Petrous Bone, even 

 into the Occipital Condyle and the root of the Zygoma. The chief Accessory 

 Cavity is the Mastoid Antrum (as large as a French Bean) with its long axis 

 (^/5th inch) corresponding to the axis of the EUSTACHIAN Tube and opens into 

 the Tympanic Cavity on the posterior wall, directly below the Tegmen Tympani 

 which forms the roof of the Antrum. The Mastoid Cells, which vary considerably 

 in number, either few or large, numerous or small open into the Antrum. The 

 cells extending into the Squamous Portion are known as Squamosal Cells, but 

 these never extend higher than the temporal ridge. The lamina of bone covering 

 the Mastoid Process is so variable in thickness that, like the Tegmen it is deficient 

 at some points on its outer wall as well as on its inner wall, which is in relation 

 to the Lateral Sinus. These points of deficiency are merely covered by connec- 

 tive tissue. 



Pus can spread easily through the thin Tegmen Tympani and cause 

 Meningitis or a Temporal Abscess. Fig. 23 shews the Vth Cranial Nerve in 

 Meckel's Cavity, i. e. the depression on the superior surface of the Petrous Bone 

 covered in by Dura Mater. 



In Fig. 24 the red line indicates the "mucous membrane axis", i. e. 

 Eustachian Tube, Tympanic Cavity, Antrum and Mastoid Cells on a line which 

 runs backwards and outwards. 



The yellow line is the "sensory axis" which passes tlirough the External 

 Auditory Meatus, Tympanic Membrane and Cavity, Vestibule and Internal Auditory 

 Meatus. These axes cross in the Tympanic Cavity ; the first passes through all 

 organs of accessory importance for hearing; the second passes through the organs 

 of hearing proper. 



