STRUCTURE OF THE MEMBRANA TYMPANI. 



749 



different conditions of atmospheric pressure within and without the tympanic 

 cavity. Its form is nearly circular ; and it has a diameter in the adult, 

 according to Sappey, of a little more than f of an inch (10 to 11 mm.) verti- 

 cally and about % of an inch 

 (10 mm.) antero-posteriorly. 

 The excess of the vertical 

 over the horizontal diame- 

 ter is about -fa of an inch 

 (0-5 mm.) 



The periphery of the 

 tympanic membrane is re- 

 ceived into a little ring of 

 bone, which may be sepa- 

 rated by maceration in early 

 life, but which is consoli- 

 dated with the adjacent 

 bony structures in the adult. 

 This bony ring is incom- 

 plete at its superior portion, 

 but aside from this, it re- 

 sembles the groove which 

 receives the crystal of a 

 watch. At the periphery 

 of the membrane, is a ring 

 of condensed, fibrous tissue, 

 which is received into the 

 bony ring. This ring also 

 presents a break at its supe- 

 rior portion. 



The concavity of the 

 membrana tympani presents 

 outward, and it may be in- 

 creased or diminished by 

 the action of the muscles of the middle ear. The point of greatest concav- 

 ity, where the extremity of the handle of the malleus is attached, is called 

 the umbo. Upon the inner surface of the membrane are two pouches, or 

 pockets. One is formed by a small, irregular, triangular fold, situated at the 

 upper part of its posterior half and consisting of a process of the fibrous 

 layer. This, which is called the posterior pocket, is open below and extends 

 from the posterior upper border of the membrane, to the handle of the mal- 

 leus, which it assists in holding in position. " After it has been divided, the 

 bone is much more movable than before " (Troltsch). The anterior pocket 

 is lower and shorter than the posterior. It is formed by a small, bony pro- 

 cess turned toward the neck of the malleus, by the mucous membrane, by the 

 bony process of the malleus, by its anterior ligament, the chorda tympani 

 and the anterior tympanic artery. The handle of the malleus is inserted 



49 



FIG. 268. Right membrana tympani, seen from within. From 

 a photograph, and somewhat reduced (Rudinger). 



1, head of the malleus, divided ; 2, neck of the malleus : 3, han- 

 dle of the malleus, with the tendon of the tensor tympani 

 muscle ; 4, divided tendon of the tensor tympani ; 5, 6, por- 

 tion of the malleus between the layers of the membrana 

 tympani ; 7, outer (radiating) and inner (circular) fibres of 

 the membrana tympani ; 8, fibrous ring of the membrana 

 tympani; 9, 14, 15, dentated fibres, discovered by Gruber; 10, 

 posterior pocket; 11, connection of the posterior pocket with 

 the malleus; 12, anterior pocket; 13, chorda tympani nerve. 



