452 TYMPANUM CONTENTS. 



TYMPANUM. The tympanum is an irregular bony cavity, com- 

 pressed from without inwards, and situated within the petrous bone. 

 It is bounded externally by the meatus and membrana tympani ; in- 

 ternally by its inner wall ; and in its circumference by the petrous 

 bone and mastoid cells. 



The Membrana tympani is a thin and semi-transparent membrane 

 of an oval shape, the longer diameter being nearly vertical. It is 

 inserted into a groove around the circumference of the meatus near 

 to its termination, and is placed obliquely across the area of that 

 tube, the direction of the obliquity being downwards and inwards. 

 It is concave towards the meatus, and convex towards the tympa- 

 num, and is composed of three layers, an external cuticular, middle 

 fibrous and muscular, and internal mucous, derived from the mucous 

 lining of the tympanum. 



The tympanum contains three small bones, ossicula auditus, viz. 

 the 



Malleus, 



Incus, 



Stapes. 



The Malleus (hammer) consists of a head, neck, handle (manu- 

 brium), and two processes, long (processus gracilis), and short (pro- 

 cessus brevis). The manubrium is connected to the membrana 

 tympani by its whole length extending to below the central point 

 of that membrane. It lies beneath its mucous layer, and serves as 

 a point of attachment to which the radiating fibres of the fibrous 

 layer converge. The long process descends to a groove near to 

 the fissura Glaseri, and gives attachment to the laxator tympani 

 muscle. Into the short process is inserted the tendon of the tensor 

 tympani, and the head of the bone articulates with the incus. 



The Incus (anvil) is named from an imagined resemblance to an 

 anvil. It has' also been likened to a bicuspid tooth, having one root 

 longer, and widely separated from the other. It consists of two 

 processes, which unite nearly at right angles, and at their junction 

 form a flattened body, to articulate with the head of the malleus. 

 The short process is attached to the margin of the opening of the 

 mastoid cells by means of a short ligament; the long process 

 descends nearly parallel with the handle of the malleus, and curves 

 inwards, near to its termination. At its extremity is a small 

 globular projection, the os orbiculare, which is a distinct bone in the 

 foetus, but becomes anchylosed to the long process of the incus in 

 the adult ; this process articulates with the head of the stapes. 



The Stapes is shaped like a stirrup, to which it bears a close re- 

 semblance. Its head articulates with the os orbiculare, and the two 

 branches are connected by their extremities with a flat oval-shaped 

 plate, representing the foot of the stirrup. The foot of the stirrup is 

 received into the fenestra ovalis, to the margin of which it is con- 

 nected by means of a circular ligament ; it is in contact, by its sur- 



