442 NERVOUS SYSTEM. 



the orifice of the mastoid cells, and is much shorter than the 

 other. The inferior branch is long, upright, tapering, and near- 

 ly parallel with the manubrium of the malleus, but somewhat 

 within it. 



The Orbiculare is a very small flattened sphere of bone, which 

 articulates with the lower end of the long process of the incus, 

 and in adult life is most generally fused into it, so as to lose its 

 distinctive character : the latter change sometimes occurs even 

 in early infancy. 



The Stapes is the last of the chain. It resembles very strong- 

 ly the common stirrup iron, from whence its name, and is placed 

 horizontally at right angles to the incus, being separated from 

 the extremity of the long process of the latter by the os orbicu : 

 lare, and being directed inwards to the foramen ovale. It is 

 composed of a head, two crura, and. a base. 



The head is oblong and flattened: it has a slight depression 

 where it joins the orbiculare. The crura are slightly curved, 

 with the concavities towards each other: the anterior is some- 

 what straighten than the posterior, and is also shorter. They 

 are both excavated, longfludinally, on their concave surfaces, 

 and between them is stretched a process of the lining membrane 

 of the tympanum, The base is precisely adapted to the fenestra 

 ovalis, and is connected to it by the lining membrane of the 

 tympanum, but not so closely as to prevent it from executing 

 slight vibratory movements. 



Between the malleus and the incus there is a moveable arti- 

 culation with a synovial membrane, but the other joints of the 

 chain are simply ligamentous. 



This chain of bones is moved by several muscles, which in- 

 fluence the degree of tension of the membrana tympani. 



1. The Laxator Tympani arises from the posterior end of 

 the spinous process of the sphenoid bone, and passing behind 

 the articulation of the lower jaw into the glenoid foramen, is 

 inserted, tendinous, along the processus gracilis of the malleus. 



It draws the malleus forwards and outwards, so as to relax 

 the membrana tympani. 



