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 to 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 with 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 vestibuli. 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 
or fenestra cochlee, 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 
861 
