THE ORGAN OF HEARING. 



intermingle, forming irregularly disposed bundles and trabeculae, 

 the dendritic fibrous structures of Gruber. 



The mucous layer of the tympanic membrane consists of sim- 

 ple squamous epithelium separated from the lamina propria by a 

 thin connective-tissue layer containing but few cells. It likewise 

 extends over the handle of the malleus. In the flaccid portion of 

 the tympanic membrane the lamina propria disappears, so that in 

 this region the cutaneous layer and the mucous membrane are in 

 direct contact. 



B. THE MIDDLE EAR. 



The middle ear, or tympanum, is a small irregular cavity, filled 

 with air, situated in the petrous portion of the temporal bone be- 

 tween the bony wall of the inner ear and the tympanic membrane, 

 and communicates with the pharynx through the Eustachian tube. 

 It contains the small bones of the ear, their ligamentous attach- 

 ments, and, in part, the muscular apparatus moving them. 



The mucous membrane lining the tympanic cavity is folded over 

 the ossicles and ligaments of the tympanum and is joined to that of the 

 tympanic membrane and the Eustachian tube, the line of junction 

 with the former being marked by the presence of papilla-like eleva- 

 tions. 



The epithelium of this mucous membrane is a simple pseudo- 

 stratified ciliated epithelium, having two strata of nuclei. Cilia are, 

 however, lacking on the surface of the auditory ossicles, on their 

 ligaments, and on the promontory of the inner wall, as well as on the 

 tympanic membrane. The mucosa of the mucous membrane is 

 intimately connected with the periosteum, and may now and then 

 contain short isolated alveolar glands, especially in the neighbor- 

 hood of the opening of the Eustachian tube. 



The " auditory ossicles" are true bones with Haversian canals 

 and lamellae ; with the exception of the stapes, they contain no 

 marrow-cavity. Very distinct perivascular spaces are seen sur- 

 rounding the vessels in the canals (Rauber). The malleus articu- 

 lates with the incus, both articular surfaces being covered with 

 hyaline cartilage. Within this articulation we find a fibrocartilagin- 

 ous meniscus, and at the summit of the short limb of the incus 

 another small cartilage plate. Between the lenticular process of the 

 incus and the capitulum of the stapes is another articulation, also 

 provided with cartilaginous articular surfaces. The basal plate of 

 the stapes is covered both below and at its edges with cartilage, as 

 are also the margins of the fenestra ovalis (fenestra vestibuli). The 

 basal plate is held in place within the fenestra by an articulation, 

 provided with tense ligamentous structures on the tympanic and 

 vestibular sides. Between these the connective tissue is quite loose. 

 All the cartilaginous portions of the auditory ossicles, with the ex- 



