THE EAR 1547 



Summary of Important Structures closely related to the Mastoid 



Antrum. 



r. Temporal lobe of cerebrum and its meninges (roof). 



2. Genu and descending limb of sigmoid sinus, and farther back the cere- 

 bellum 'posterior wall . 



3. Facial nerve (posterior wall of cavum tympani. close to inner wall, and 

 nearly flush with antral opening^ 



4. External semicircular canal (anterior part of inner wall). 



The upper part of the antrum communicates, as stated, with the 

 attic of the cavum tympani, but the lower part is shut off by bone 

 from the cavity of the ca\'iun t^mpanL 



The mucous membrane of the mastoid antrum is continuous 

 with that which hnes the mastoid cells. Anteriorly it is also con- 

 tinuous, as stated, with the mucous membrane of the attic and 

 ca\Tmi t\Tnpani or tympanum proper. The mucous membrane of 

 the caviun t\Tnpani is continuous anteriorly with that of the 

 Eustachian tube, and the mucous membrane of the Eustachian tube 

 is continuous with that of the naso-pharynx. This extensive 

 and continuous tract of mucous membrane is covered by columnar 

 ciliated epitheliimi, except in the following regions: (i) the pro- 

 montory; (2) the t^onpanic ossicles; (3) the membrani tympani; 

 ^4) the mastoid antrum ; and (5) the mastoid cells. In these regions 

 the epithelium consists of a single layer of squamous non-ciliated 

 cells. 



It is of considerable importance to note that micro-organisms 

 may pass from the naso-pharynx through the Eustachian tube into 

 the cavum tympani and attic, and thence into the mastoid antrum 

 and mastoid cells. Purulent affections of these regions may there- 

 fore readily be caused in this manner. Such affections may sub- 

 sequently involve (i) the temporal lobe of the cerebrum and its 

 meninges, (2) the genu and descending limb of the sigmoid sinus, 

 and (3) the internal ear or labyrinth. 



Arteries of the Tympanum. — The principal £u-teries are two in number, 

 namely, (i) the tympanic branch of the internal maxillary, and {2) the stylo- 

 mastoid branch of the posterior auricular. The tympanic artery enters 

 through the fissure of Glziser, and supplies the membrana tympani and front 

 part of the tympanum. The stylo-mastoid artery enters the aqueduct of 

 Fallopius through the stylo-mastoid foramen, and passes from the descending 

 part of the aqueduct into the tympanum. It supplies the back part of the 

 cavity and the mastoid cells, and it forms, with the tympanic artery, a ring 

 round the circumference of the membrana tympani. 



In addition to the foregoing two arteries, the following three £irteries enter 

 the tympanic cavity : (i) the petrosal branch of the middle meningeal, which 

 enters from the aqueduct of Fallopius, into which it passes through the hiatus 

 Fallopii ; {2) the tympanic branch of the ascending pharyngeal, which accom- 

 panies Jacobson's nerve through the tympanic canaliculus ; and (3) the 

 tympanic branch of the internal carotid, which enters by a minute foramen 

 on the posterior wall of the ascending part of the carotid cancil, in company 

 with a sympathetic twig from the carotid plexus. 



The veins of the tympanum pass to the pterygoid plexus, the superior 

 petrosal sinus, the internal jugular vein, and the pharyngeal plexus. 



