THE ORGAN OF HEARING. 



IX. THE ORGAN OF HEARING. 



THE ear, the organ of hearing, consists of three parts : (i) The 

 external ear, including the pinna or auricle and the external audi- 

 tory- canal ; (2) the middle ear, tympanum, or tympanic cavity, 

 containing the small ear bones and separated from the external 

 auditory canal by the tympanic membrane, but communicating with 

 the pharynx by means of the Eustachian tube ; (3) the inner ear, 

 or labyrinth, consisting of a bony and a membranous portion, the 

 latter lined by epithelial cells, especially differentiated in certain 

 regions to form a neuro-epithelium, in which the auditory nerves 

 terminate. The first two parts serve for the collection and trans- 

 mission of the sound-waves ; the complicated labyrinth, with its 

 differentiated neuro-epithelium, for the perception of the same. 

 Figure 366 presents in a schematic way the relationships of the 

 parts here mentioned. 



A. THE EXTERNAL EAR. 



The cartilage of the ear, including that of the external auditory 

 passage, is of the elastic variety, but differs from typical elastic carti- 

 lage in that it contains areas entirely free from elastic fibers. The 

 elastic reticulum is, however, never absent near the perichondrium. 

 The skin covering the pinna is thin, and in it are found hairs with 

 relatively large sebaceous glands ; sweat-glands are found on the 

 outer surface. 



The skin lining the cartilaginous portion of the external auditory 

 canal is somewhat mobile and possesses very few pronounced 

 papillae, and is characterized by the presence of so-called ceruminous 

 glands, which represent modified and very highly differentiated 

 sweat-glands. They are branched, tubulo-alveolar glands (Huber). 

 They empty either into the hair follicles near the surface of the skin 

 or on to the surface of the skin in the neighborhood of the hair fol- 

 licles. 



The skin lining the osseous portion of the external auditory 

 canal is supplied with neither hair nor glands, and possesses slender 

 papillae, especially in the neighborhood of the tympanic membrane. 

 The corium is closely attached to the periosteum. 



The tympanic membrane consists of a tense and a flaccid portion. 

 It forms a part of both the external and the middle ear. From 

 without inward, the following layers may be differentiated : (i) the 

 cutaneous layer ; (2) the lamina propria ; and (3) the mucous layer. 



The epidermis of the cutaneous layer is identical in structure 

 with that of the outer skin, except that the superficial layers of the 

 stratum corneum contain nucleated cells. The corium is very thin, 

 except along the course of the ma'nubrium of the malleus, where it 



