ANATOMY IN A NUTSHELL. 571 



The membrana tympani, which is attached to a grooved ridge of bone at 



the bottom of the external auditory meatus, is an oval, clastic semi-trans- 

 parent membrane about 7 ,] of an inch in thickness, aboul ,-'•., of an inch 

 in its antero-posterior diameter and slightly less in its vertical diameter. This 

 membrane is situated obliquely, its outer surface being directed outward, 

 downward, and forward, so as to form almost a continuation of the posterior 



wall of the external auditory meatus. In infancy this membrane is al >t 



horizontal. The membrana tympani is composed of three Layers; an outer 

 or cutaneous layer, a middle or fibrous layer, and an inner or mucous layer. It 

 has the following points for consideration: 



1. The umbo (naval) is the dark depressed center of the membrane. 



2. The cone of light is a triangular area, the apex of which i> attached to 

 the tip of the handle of the malleus, and at its base extends toward the circum- 

 ference of the membrane. This is of value hi diagnosis of diseases of the tym- 

 panum and membrana tympani. 



3. The tubercle which is at the upper border is formed by theshorl pro- 

 cess of the malleus. 



4. The stripe which runs down from the tubercle to the umbo and is formei 1 

 by the handle of the malleus. 



5. Sharpnell's membrane (membrana fiaccida) is situated at the upper 

 part of the membrane below the notch of Rivini. This notch is formed where 

 the bony ring to which the tympanic membrane is attached is incomplete. 



6. Rivinian foramen is a minute opening which exists in Sharpnell's mem- 

 brane. 



7. Membrana tensa is all the tympanic membrane except the membrana 

 fiaccida. This membrane is concave externally. 



Blood Supply. — From tympanic branches of the internal maxillary and 

 carotid arteries. 



Nerve Supply. — The external surface receives the auriculo-temporal from 

 the fifth nerve and Arnolds' nerve from the tenth. The internal surface receives 

 branches from the tympanic plexus. 



LESSON CXCIX. 

 The Middle Ear. 



The tympanum (drum) is an irregular cavity situated within the petrous 

 portion of the temporal bone, and lined with mucous membrane. It lies be- 

 tween the external auditory meatus and the internal ear. lis antero-posterior 

 length is about half of an inch, vertically a lil tie more I ban half an inch, and its 

 width is about one sixth of an inch. It consists of two main parts, (a) attic or 

 recessus epitympaniticus which is situated in the highesl portion of the tym- 

 panic cavity and contains the head of themalleus and the greaterparl of the 

 incus. This attic leads into the mastoid antrum, do Atrium (tympanic. cav- 

 ity proper) is situated opposite the tympanic membrane. 



The roof of the tympanum is a thin plate of bone (tegmen tympani) which 

 separates the tympanum from the middle fossa :it the base of the skull. 



