ACTION OF THE MUSCLES. LINING MEMBRANE. 751 



stantly in the horse, the ox, and other animals. This circumstance is the more inte- 

 resting when it is remembered that cartilage occupies the position of the stapedius 

 before the muscle is developed. (P. 66 and fig. 528.) 



Actions. The malleus and incus move together round an axis extending backwards 

 from the attachment of the processus gracilis of the malleus in the Glasserian fissure 

 to the attachment of the short process of the incus posteriorly. The tendon of the 

 tensor tympani muscle passing from within to be inserted below that line, pulls the 

 handle of the malleus inwards, while the laxator tympani inserted above that line, by 

 pulling the head of the bone inwards, moves the handle outwards. The incus, moving 

 along with the malleus, pushes the stapes inwards towards the internal ear when the 

 membrana tympani is made tight, and withdraws that bone from the fenestra ovalis, 

 when the membrana tympaui is relaxed. But the cavity of the inner ear is full of 



Fig. 505. OUTLINE OF THE THREE SMALL BONES OF 

 THE LEFT EAR AS SEEN FROM BEFORE. f 



This figure is designed to illustrate the effect of 

 the action of the tensor and laxator muscles of the 

 tympanic membrane in connection with their relation 

 to the axis of rotation of the malleus, a, a', the 

 malleus ; 6, the incus seen behind it ; c, the stapes ; 

 m, mf, the inner part of the meatus externus closevl 

 by the tympanic membrane, of which the posterior 

 half is represented ; the axis of rotation of the 

 malleus being supposed to pass through a point at 

 the root of the processus gracilis, g ; the line t, indi- 

 cates the direction and position of the tendon of the tensor tympani pulling the lower 

 part of the malleus inwards, the line /, that of the laxator tympani pulling inwards the 

 upper half of the malleus. 



liquid ; and its walls are unyielding, except at the fenestra rotunda ; when, there- 

 fore, the stapes is pushed inwards the secondary membrane of the tympanum, which 

 blocks up the fenestra rotunda, must be made tense by pressure from within. The 

 attachment of the handle of the malleus, however, to the membrana tympani allows 

 greater freedom of movement to that process than is allowed to the stapes by the 

 ligament of its base, and when the movement of the stapes ceases, it is plain that the 

 malleus in any movement must rotate on the head of the incus; and hence, probably, 

 the necessity of a moveable articulation between tho*e bones. The action of the 

 stapedius muscle is obviously to draw the head of the stapes backwards, in doing which 

 the hinder end of the base of that bone will be pressed against the margin of the 

 fenestra ovalis, while the fore part will be withdrawn from the fenestra. The object 

 gained by this movement of the stapes is not sufficiently ascertained; but it is at least 

 evident that, if the stapes be pressed inwards by the incus in the action of the tensor 

 tympani, the stapedius muscle, if then contracted, will modify the pressure on the 

 internal ear. It is conceivable that the stapedius may thus protect the sensitive part 

 of the ear to a certain extent from excessive stimulation of the auditory nerve. 



THE LINING MEMBRANE OF THE TYMPANUM. 



The raucous membrane of the tympanum is continuous with that of the 

 pharynx through the Eustachian tube, and is further prolonged from the 

 tympanum backwards into the mastoid cells. Two folds which cross the 

 breadth of the cavity descend from the part of the membrane which lines 

 the roof. The anterior fold descends to turn round the tendon of the tensor 

 tympani muscle ; the posterior fold passes round the stapes. The malleus 

 and incus are invested by the lining of the outer wall of the cavity. The 

 mucous membrane which lines the cartilaginous part of the Eustachian tube 

 resembles much the membrane of the pharynx, with which it is immediately 

 continuous ; it is thick and vascular, and is covered by several layers of 

 laminar epithelium with vibratile cilia, and is provided with many simple 

 mucous glands which pour out a thick secretion : in the osseous part of the 



