MALLEUS-INCUS ARTICULATION 



271 



K>.6 



Scale 5mm 



Fig. VII-9. Scale detailed structure of the incus (/) and attached stapes (S) 

 with footplate inserted into its oval window. Note round window (R. W.) in the 

 acoustic shadow of waves that may originate from vibrating ossicles and tympani. 

 The section through the cochlea is taken just posterior to the axis of the modiolus. 

 The acoustic nerve enters the modiolus at N. View of left ear with the section con- 

 taining the malleus and tympani, whose edges are shown, removed. Figure VII-8 

 may be superimposed on this figure to give correct position of the ossicles. (Stuhl- 

 man.) 



Mechanical Advantage of the Malleus-Incus Articulation 



The malleus has a figure-8 convex articular surface, covered with cartil- 

 age, and the incus a corresponding concave surface; a capsule encloses 

 these surfaces. Figure VII-7 shows these two surfaces with their liplike 

 ridges, which are mirror images of each other. The ridges are higher at 

 H and lower at L. The two rounded surfaces fit into each other like a 

 shallow ball-and-socket joint and make close contact when the muscles 

 are relaxed. This close-fitting position will be referred to as the locked 

 or close-coupled position of the malleus-incus articulation. 



In this locked position of the malleus-incus joint the manubrium 

 lever arm is to the incus lever arm as 1.27 to 1. If the malleus-incus joint 

 is coupled so that separation can take place, two centers of rotation de- 



