424 PRACTICAL ANATOMY. 



THE MIDDLE EAR. 



The middle ear, or tympanum, is an irregular, cu- 

 boidal cavity, situated within the petrous portion of the 

 temporal bone. It is separated from the auditory canal 

 by the tympanic membrane. It is traversed by a chain 

 of bones, which convey the vibrations of sound from the 

 tympanic membrane to the internal ear, and is filled with 

 air and communicates with the pharynx by the Eustachian 



FIG. 184. LEFT TYMPANUM AND AUDITORY OSSICL.ES. 



A. G., external meatus ; M. memhrana tympani. which is attached to the handle of the 

 malleus, n, and near it the short process, p ; h, head of the malleus ; a, incus : K, its short 

 process, with its ligament; 1, long process; s, Sylvian ossicle; S. stapes; A x, A x, the 

 ax s of rotation of the ossicles, shown in perspective : t, line of traction, of the tensor tym- 

 pani. The other arrows show the movements of the ossicles when the tensor contracts. 



tube. The cavity of the tympanum is about one-sixth 

 inch in depth, one-fourth inch vertically, and three- 

 eighths inch antero-posteriorly. It is irregularly cu- 

 boidal, and presents six walls for examination. The outer 

 is formed by the membrana tympani, which is attached 

 to a ring of bone. It presents three apertures, the 

 anterior and posterior, for the transmission of the chorda 

 tympani nerve, and the Glaserian fissure. The posterior 



