ORGANS OF SENSE. 155 



supplied by a mere ring. The auditory canal is lined by the integument, 

 continued from the auricle ; the cutis ceases at the margin of the lower 

 extremity, but the cuticle is continued over the membrana tympani. The 

 skin, from a little distance within the external orifice ig within a quarter of 

 an inch of the tympanum, is perforated by numerous small openings, the 

 openings of the sebaceous follicles and of the ceruminous glands. These 

 glands secrete the cerumen, or ear wax. 



B. Middle Ear. 



The middle ear or tympanum is the space within the membrana tympani, 

 external to the osseous walls of the vestibule and labyrinth, in front of the 

 mastoid cells, and communicates anteriorly with the Eustachian tube. It 

 contains air and a little fluid, and is traversed by the chain of small bones, 

 and by fine nerves and vessels. Its walls are lined by a fine mucous mem- 

 brane, continuous through the Eustachian tube Avith that of the nose. 



1. Membrana Tympani. This is situated at the inner and lower extre- 

 mity of the meatus, separating it from the cavity of the tympanum or drum, 

 the outer wall of which it forms. It is a thin, dry, semi-transparent mem- 

 brane, nearly circular, with the vertical diameter a little longer than the 

 transverse or antero-posterior. The inner surface of the membrana is 

 directed upwards and inwards, and adheres intimately to the handle of the 

 malleus bone. Although thin and semi-transparent, the membrana tympani 

 consists of three distinct layers, an external or epidermis, an internal or 

 mucous, and a middle or fibrous. We have already referred to the two 

 former ; the latter presents the appearance of fibres radiating from the 

 exterior to the malleus, into which they are inserted. 



The internal or vestibular side of the tympanum presents several points 

 of observation. At its upper part is situated the kidney -shaped foramen, 

 fenestra ovalis or vestibuU. This is directly opposite the membrana tym- 

 pani, and in the dry bone leads to the vestibule ; in the recent state it is 

 closed by a membrane. The base of the stapes also closes this opening. 

 Below the foramen ovale is the bony projection of the promontory^ which 

 corresponds to the first turn of the cochlea, and on the surface of which 

 there are delicate grooves converging below to a common canal, which 

 transmits the nerve of Jacobson from the glosso-pharyngeal nerve. Behind 

 the fenestra ovalis is the bony pyramid, the apex of which is perforated by 

 a small canal. Below and behind the promontory is the foramen rotundum 

 OT fenestra cochlece, which in the recent state is closed by membrane. 



The posterior wall of the tympanum presents a large irregular opening at 

 its upper part, leading into the mastoid cells. The anterior wall tapers off 

 into the Eustachian tube, above which are the cochleariform osseous plate 

 and the canal for the tensor tympani muscle. 



2. The Eustachian Tube is a narrow canal, about an inch and a half 

 long, leading from the tympanum obliquely towards the back of the nose, 

 where it opens on the side of the pharynx and on a level with the inferior 

 spongy bone. The tympanic portion is chiefly osseous ; the remainder is 

 composed of cartilaginous, fibrous, and mucous tissues. The use of this tube 



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