234 THE NERVOUS SYSTEM. 



External ear : The auricle consists of a basis of elastic 

 cartilage covered with subcutaneous tissue and skin except 

 the lobule, whose interior is occupied by fibrous tissue. 



The outer (cartilaginous) portion of the external auditory 

 canal is lined with skin, which is very thick and contains 

 hairs, their sebaceous glands, and ceruminous glands, a modi- 

 fied form of sweat-gland. This portion of the canal has a 

 basis of elastic cartilage beneath the subcutaneous tissue. 



The inner (bony) portion of the auditory canal is mostly 

 lined with thin skin containing no glands or hair. 



The middle ear (or tympanum) is a cavity filled with air, 

 communicating with the Eustachian tube anteriorly and the 

 mastoid sinuses posteriorly, and crossed by a chain of ossicles 

 connecting the tympanic membrane with the fenestra ovalis. 



Its walls are formed by bone, the tympanic membrane, and 

 the secondary tympanic membrane, all lined with mucous 

 membrane. 



The mucous membrane is thin and closely connected with 

 the underlying periosteum ; it is reflected over the surface of 

 the ossicles and mastoid sinuses and is continuous with the 

 mucous lining of the Eustachian tube. Its superficial layer 

 consists of low columnar epithelium, in places ciliated, in 

 other places non-ciliated. Anteriorly are scattered tubular 

 glands. 



The tympanic membrane is made up of an intermediate 

 fibrous lamina covered externally with skin and internally 

 with a portion of the mucosa lining the middle ear; the sur- 

 face epithelium-cells of the latter are low and non-ciliated. 



The secondary tympanic membrane closes the fenestra 

 rotunda, and consists of an intermediate fibrous lamina, 

 covered externally with mucous membrane (having non-ciliated 

 low epithelium) continuous with the mucosa of the middle ear, 

 and internally with endothelium and subendothelial tissue 

 continuous with that lining the internal ear. 



The Eustachian tube is an open canal connecting the middle 

 ear with the pharynx. Its walls are formed of bone, cartilage 

 (partly hyaline, partly elastic), and fibrous tissue, lined with 

 mucous membrane continuous with that of both the middle 

 ear and pharynx. The surface cells of this mucosa are strati- 

 fied ciliated epithelium. In its upper portion the mucous 



