918 



THE ORGANS OF SENSE. 



Epitympanic recess 



Proc. cochlearif. 



Anterior watt 



rotico-tymp. canal 



Floor 



Prorrcontor. 

 Aq. Fallop. 



FIG. 543. Antero-posterior section through 

 the tympanum. (Gegenbaur.) 



i 



FIG. 544. Transverse section 

 of the Eustachiau tube. a. 

 Above, b. At about its middle. 

 c. At its lower part. 



The osseous portion is about half an inch in length. It commences in the lower 

 part of the anterior wall of the tympanum, below the processus cochleariformis, 



and, gradually narrowing, terminates in an 

 oval dilated opening at the angle of junction 

 of the petrous and squamous portions, its ex- 

 tremity presenting a jagged margin which 

 serves for the attachment of the cartilaginous 

 portion. 



The cartilaginous portion, about an inch 

 in length, is formed of a triangular plate of 

 elastic fibro.-cartilage, curled upon itself, an 

 interval being left below, between the margins 

 of the cartilage, which is completed by fibrous 

 and muscular tissue. Its canal is narrow 

 behind, wide, expanded, and somewhat trum- 

 pet-shaped in front, terminating by an oval 

 orifice at the upper part and side of the pharynx, behind the back part of the infe- 

 rior meatus. Through this canal the mucous membrane of the pharynx is contin- 

 uous with that which lines the tympanum. The mucous 

 membrane is covered with ciliated epithelium (Fig. 544). 

 The membrana tympani separates the cavity of the 

 tympanum from the bottom of the external meatus. It 

 is a thin, semi-transparent membrane, nearly oval in 

 form, somewhat broader above than below, and directed 

 very obliquely downward and inward. Its circumfer- 

 ence is contained in a groove at the inner end of the 

 meatus, which skirts the circumference of this part, ex- 

 cepting above. The portion filling in the notch of Rivinus (see above) is looser 

 in texture than the remainder, and is known as the membrana flaccida. The 

 handle of the malleus descends vertically between the inner and middle layers of 

 this membrane as far down as its centre, where it is firmly attached, drawing the 

 membrane inward, so that its outer surface is concave, its inner convex. The 

 middle of the concavity is known as the umbo. 



Structure. This membrane is composed of three layers, an external (cuticular), 

 a middle (fibrous), and an internal (mucous). The cuticular lining is derived from 

 the integument lining the meatus. The fibrous layer consists of fibrous and elastic 

 tissues ; some of the fibres radiate from near the centre to the circumference ; others 

 are arranged, in the form of a dense circular ring, round the attached margin of 

 the membrane. The mucous lining is derived from the mucous lining of the 

 tympanum. The vessels pass to the membrana tympani along the handle of the 

 malleus, and are distributed between its layers. 



Ossicles of the Tympanum (Fig. 545). 



The tympanum is traversed by a chain of movable bones three in number, the 

 malleus, incus, and stapes. The former is attached to the membrana tympani, the 

 latter to the fenestra ovalis, the incus being placed between the two, to both of 

 which it is connected by delicate articulations. 



The Malleus, so named from its fancied resemblance to a hammer, consists of 

 a head, neck, and three processes the handle or manubrium, the processus gracilis, 

 and the processus brevis. 



The head is the large upper extremity of the bone ; it is oval in shape, and 

 articulates posteriorly with the incus, being free in the rest of its extent. 



The neck is the narrow contracted part just beneath the head, and below this 

 is a prominence to which the various processes are attached. 



The manubrium is a vertical process of bone which is connected by its outer 

 margin with the membrana tympani. It decreases in size toward its extremity, 

 where it is curved slightly forward, and flattened from within outward. On the 



