510 MUSCLES OF THE TYMPANUM. 



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 than, 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 

 its termination. At its extremity is a small globular projection, the 

 os orbiculare, which in the foetus is a distinct bone, 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 resem- 

 blance. 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- 

 face, with the membrana vestibuli, and is covered in by the mucous 

 lining of the tympanum. The neck of the stapes gives attachment to 

 the stapedius muscle. 



The ossicula auditus are retained in their position and moved upon 

 themselves by means of ligaments and muscles. 



The Ligaments are three in number ; the ligament of the head of 

 the malleus, which is attached superiorly to the upper wall of the 

 tympanum ; the ligament of the incus, a short and thick band, which 

 serves to attach the extremity of the short process of that bone to the 

 margin of the opening of the mastoid cells ; and the circular ligament 

 which connects the margin of the foot of the stapes with the circum- 

 ference of the fenestra ovalis. These ligaments have been described 

 as muscles, by Mr. Tod, under the names of superior capitis mallei, 

 obliquus incudis externus posterior, and musculus vel structura sta- 

 pedii inferior. 



The Muscles of the tympanum are four in number, the 



Tensor tympani, 

 Laxator tympani, 

 Laxator tympani minor, 

 Stapedius. 



The Tensor tympani (musculus internus mallei) arises from the 

 spinous process of the sphenoid, from the petrous portion of the tem- 

 poral bone, and from the Eustachian tube, and passes forwards in a 

 distinct canal, separated from the tube by the processus cochleariformis, 

 to be inserted into the handle of the malleus, immediately below the 

 commencement of the processus gracilis. 



The Laxator tympani (musculus externus mallei) arises from the 



