580 THE ORGANS 



by the surrounding bony structures covered by periosteum. It is 

 lined with mucous membrane and contains the ear ossicles and their 

 ligamentous and muscular attachments. The epithelium is of the 

 simple low cuboidal type. In places it may be ciliated and not infre- 

 quently assumes a pseudostratified character with two layers of 

 nuclei. Beneath the epithelium is a thin stroma which contains some 

 diffuse lymphoid tissue and blends with the dense underlying perios- 

 teum. Small tubular glands are usually present, especially near the 

 opening of the Eustachian tube. 



The fenestra rotunda is covered by the secondary tympanic mem- 

 brane. This consists of a central lamina of connective tissue covered 

 on its tympanic side by part of the mucous membrane of that cham- 

 ber, on the opposite side by a single layer of endothelium. 



The ossicles are composed of bone tissue arranged in the usual 

 systems of lamellae. The stapes alone contains a marrow cavity. 

 Over their articular surfaces the ossicles are covered by hyaline 

 cartilage. 



The Eustachian Tube. — This is a partly bony, partly cartilaginous 

 canal lined with mucous membrane. The epithelium of the latter is 

 of the stratified columnar ciliated variety consisting of two layers of 

 cells. In the bony portion of the tube the stroma is small in amount 

 and intimately connected with the periosteum. In the cartilaginous 

 portion the stroma is thicker and contains, especially near the pharyn- 

 geal opening, lymphoid tissue and simple tubular mucous glands. 



The Internal Ear 



The internal ear consists of a complex series of connected bony 

 walled chambers and passages containing a similar-shaped series of 

 membranous sacs and tubules. These are known, respectively, as the 

 osseous labyrinth and the membranous labyrinth. Between the two is 

 a lymph space, which contains the so-called perilymph, while within 

 the membranous labyrinth is a similar fluid, the endolymph. 



The bony labyrinth consists of a central chamber, the vestibule, 

 from which are given off the three semicircular canals and the cochlea. 

 The vestibule is separated from the middle ear by a plate of bone in 

 which are two openings, the fenestra ovalis and the fenestra rotunda. 

 Just after leaving the vestibule each canal presents a dilatation, the 

 ampulla. As each canal has a return opening into the vestibule and 

 as the anterior and posterior canals have a common return opening 



