THE TYMPANUM. 441 



separated by a very thin partition of bone, is another canal 

 which lodges a muscle of the malleus. On the outer side of 

 the Eustachian tube is the glenoid foramen, by which, in the 

 dried bone, the tympanum communicates with the glenoid ca- 

 vity; in the recent state the foramen receives the long process 

 of the malleus and its muscle, and transmits the chorda tym- 

 pani. 



There are four bones in the tympanum, which, being suc- 

 cessively articulated with each other, form a chain, one end of 

 which is fastened to the membrana tympani, and the other end 

 rests upon the foramen ovale. . They are the Malleus ; the In- 

 cus; the Orbiculare; and the Stapes. 



The Malleus forms the fore part of the chain, and is placed 

 almost vertically. Its superior extremity is the head, which is 

 rounded, with the exception of the posterior face, where a small 

 concavo-convex surface is observable, for its articulation with 

 the incus. Its lower extremity is long and tapering, inclines in- 

 wardly, terminates by a little knob, and forms an angle with 

 the part above; this portion is the manubrium, and adheres its 

 whole length to the membrana tympani, commencing at the su- 

 perior margin of the latter, and insinuating itself between the 

 internal and the proper layer, as far as the centre of the mem- 

 brane. It is this adhesion with the inclination inwards of the ma- 

 nubrium, that causes the membrane to be depressed in its centre. 



Between the head and the manubrium is a short portion called 

 the neck. From the superior external extremity of the manu- 

 brium there proceeds outwardly the short process, (Processus 

 Brevis;) and from the front of the neck, there proceeds the long 

 and very delicate process, concave externally and convex in- 

 ternally, which is insinuated into the glenoid foramen, and is 

 the Processus Longus, of Gracilis. 



The Incus is behind the malleus, and is also upright. It con- 

 sists in a body and two branches, which diverge very consider 

 rably, and has a general resemblance to a molar tooth. The 

 body presents, on its fore part, a deep concavity, which articu- 

 lates with the convex head of the malleus. The branch which 

 arises from the back part of the body is horizontal, looks into 



