THE EAR 



201 



ward spread occurs in the course of a middle ear infection, 

 and the clot may extend along the vein so that it can be felt 

 on palpation at the upper part of the anterior border of the 

 sterno-mastoid. 



The anterior wall of the middle ear is occupied, in its lateral 



FIG. 77. Section through the Auricle, the External Acoustic Meatus 

 and the Tympanum. (TURNER'S Anatomy,} 



a. Helix. 



b. Antitragus. 



c. Antihelix. 



d. Concha. 



e. Lobule. 



f. Mastoid process. 



g. Facial nerve. 



h. Styloid process. 



k. Internal carotid artery. 



/. Auditory (Eustachian) 



tube. 



ut. Apex of petrous tem- 

 poral. 



. External acoustic 

 meatus. 



o. Tympanic membrane. 



p. Tympanum. 



1. Malleus. 



2. Incus. 



3. Stapes. 



4. Cochlea. 



5. 6, 7. Semicircular canals. 



8. Facial nerve. 



9. Acoustic nerve. 



part, by the opening of the auditory (Eustachian} tube. The 

 connexion thus established with the naso-pharynx ensures 

 equality in the air pressure on the two sides of the tympanic 

 membrane. Obstruction of the auditory tube, such as occurs 

 in the condition of adenoids (p. 330), is followed by the 



