MUSCLES OF THE TYMPANUM. 459 



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, 

 which articulates with the head of the malleus. The short process is at 

 tached to the margin of the opening of the mastoid cells by means of a 

 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 fetus 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 resemblance. 

 Its head articulates with the os orbiculare, and the two branches are con- 

 nected 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 connected by means of a circular liga- 

 ment ; it is in contact, by its surface, 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 to the upper wall of the tympanum ; the liga- 

 ment of the incus, a short and thick: band, which serves to attach the ex- 

 tremity 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 circumference of the fenestra ovalis. These 

 ligaments have been described as muscles, by Mr. Tod, under the names 

 cf superior capitis mallei, obliquus incudis externus posterior, and mus- 

 culus vel structura stapedii 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 temporal 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 spinous 

 process of the sphenoid bone, and passes through an opening in the fissura 

 Glaseri, to be inserted into the long process of the malleus. This is re- 

 garded as a ligament by some anatomists. 



The Laxator tympani minor arises from the upper margin of the meatus, 

 and is inserted into the handle of the malleus, near the processus brevis. 

 This is regarded as a ligament by some anatomists. 



The Stapedius arises from the interior of the pyramid, and escapes from 

 its summit to be inserted into the neck of the stapes. 



