THE MIDDLE EAR 



877 



The ossicles of the ear are described on pages 66-6S. 

 Articulation of the Ossicles. — The malleus with the incus. 



The oblong articular surface which descends as far as the neck, on the posterior 

 surface of the head of the malleus, is divided into two portions by a nearly vertical 

 edge. The incus presents a corresponding surface similarly divided into two por- 

 tions, and both of these surfaces are coated l)y a delicate layer of hyaline cartilage. 

 A thin capsule, from the inner surface of which a delicate wedge-shaped meniscus 

 projects from above into the cavity of the joint, is attached to the grooves, Avhich 

 limit the articular surfaces. The mechanism has been compared by Helmholtz to 

 the check movement inside the key of a Geneva watch, and permits considerable 

 gliding Ijetween the two l>ones. 



The incus with the stapes. — This joint is formed by the convex surface of the 

 lenticular process (orbicular bone) which terminates the long crus of the incus 

 and the corresponding concave surface of the head of the stapes. Both surfaces 

 are covered with a layer of hyaline cartilage. 



The elastic capsular ligament permits but very slight separation between the 



Fig. 499. — Section of the Ty.aipaxum, etc. (Enlarged.) 



HEAD OF MALLEUS 



LONG CRUS OF INCUS 

 BASE OF STAPES 

 HEAD OF STAPES 



ORBICULAR BONE, OR 



LENTICULAR PROCESS 



EUSTACHIAN TUBE 



Fibrous layer of 

 membrana tympani 



CAVITY OF TYMPANU^ 



Suspensory ligament 



ATTIC OF THE TYMPANUM 



External ligament 



SHORT CRUS OF VALLEUS 



— HANDLE OF MALLEUS 



_ EXTERNAL AUDITORY CANAL 



." I , / I ; ' 



bones. Init allows a somewhat freer lateral movement. This articulation has been 

 regarded by some writers as a synchondrosis, but it is now usually described as a 

 true joint. 



The stapes with the margin of the fenestra ovalis. — The so-called annular 

 ligament is not equally l)r()ad throughout, and con-sists of elastic fibres which 

 radiate as they pass outwards from the edge of the footpiece of the stapes to the 

 margin f)f the fenestra. According to Toynbee, these edges are covered by a tliin 

 layer of cartilage. 



The Ligaments of the Ossicles. — In addition to the capsular ligaments al- 

 ready described, certain well-marked bands limit the movements of the ossicles. 



The superior ligament of the malleus descends as a firm rounded band from 

 the up|)er and outer wall of the tym])anic cavity to the head of the malleus. It 

 limits the outward movement of the linndle of tlie malleus. 



The anterior ligament of the malleus is broad and strong. According to 

 Helmholtz it encircles the long process of the malleus, and is inserted into the 

 fore part of its neck and head. 



