704 THE ORGANS OF SENSE. 
meatus, and are filled by fibrous tissue. In the outer part of the meatus, the cartilage 
forms about three-fourths of its circumference ; but, on passing inwards, the propor- 
tion of cartilage to fibrous tissue diminishes, with the result that near the inner 
end of the pars cartilaginea the cartilage forms merely a part of the anterior and 
lower boundaries of the canal. 
The osseous portion (pars ossea) of the meatus is described on p. 113; but it may be 
well to state here that in the newly-born child it is represented only by an incomplete 
ring of bone, the annulus tympanicus, together with a small portion of the squamous 
temporal, Ww hie h articulates with, and bridges over the interval between, the extremities of the 
ring superiorly. In the concavity of the annulus is a well-defined groove, the sulcus tympanicus, 
in which the circumference of the membrana tympani is fixed. A fibrous plate, the tympanic 
fibrous plate (Symington), intervenes between the annulus tympanicus and the inner end of the 
cartilage of the meatus, and into this plate the bony ring extends) The bony outgrowth does 
not, huwever, proceed uniformly throughout its circumference, but occurs most rapidly in the 
anterior and posterior parts of the ring. These outgrowths fuse about the end of the second year 
of life, so as to surround a foramen in the floor of the meatus (foramen of Huschke), which is 
usually closed by the fifteenth year, but persists until adult life in some 19 per cent. of skulls. 
The lumen of the meatus in the newly-born child is extremely small : its outer part is funnel- 
shaped ; its inner a mere slit, hounded below by the tympanic fibrous plate and above by the 
obliquely-placed membrana tympani. 
The skin which envelopes the pinna lines the entire meatus, and covers also the 
outer surface of the eres membrane. It is thick in the pars cartilaginea and 
contains fine hairs and sebaceous glands, the latter extending inwards for some dis- 
tance along the postero-superior ‘wall of the pars ossea. The sweat glands are 
enlarged and of a brownish colour; they constitute the glandule ceruminose and 
secrete the ear wax or cerumen. 
Vascular and Nervous Supply of the Meatus.—The meatus receives its blood-supply from 
the posterior auricular and superficial temporal arteries, and also from the deep auricular branch 
of the internal maxillary artery, the last distributing some minute branches to the membrana 
tympani. The veins open into the external jugular and internal maxillary veins, and also into 
the pterygoid plexus, while the lymphatics have a similar mode of termination to those of the 
pinna. Sensory nerves are supplied to the meatus by the auriculo-temporal branch of the fifth 
and by the auricular branch of the vagus. 
MIDDLE EAR OR TYMPANIC CAVITY. 
The tympanic cavity (cavum tympani) is a small air chamber in the temporal 
bone, intervening between the inner end of the external auditory meatus and the outer 
wall of the internal ear or labyrinth (Figs. 513,514). Lined by mucous membrane, 
it contains a chain of ossicles (ossicula auditus), which reaches from its outer to its 
inner wall, and by means of which the vibrations of the membrana tympani are 
transmitted across the cavity to the internal ear. Attached to the ossicles are 
several ligaments, together with a pair of small muscles, while certain nerves are 
either distributed to the cavity or pass through it. 
The tympanic cavity consists of two portions: (1) The tympanum proper, or 
atrium, lying immediately to the inner aspect of the membrana tympani; and (2) 
the recessus epitympanicus, or aditus ad antrum, lying above the level of the mem- 
brane and containing the greater part of the incus and the upper half of the 
malleus. Including “this recess, the vertical and antero-posterior diameters of the 
tympanic cavity are rather more than half an inch (15 mm.). The distance 
between its outer and inner walls is about 6 mm. above and 4 mm. below, while at 
its central part, owing to the bulging of the two walls towards the cavity, it 
measures only from 1} to 2 mm. 
The tympanic cavity presents for examination a roof, a floor, and four walls, viz. 
anterior, posterior, external, and internal. 
The roof (Fig. 515) (paries tegmentalis) is formed by a thin plate of ‘bone, the 
tegmen tympani, ‘constituting a portion of the upper surface of the petrous- temporal. 
It extends backwards so as ‘to cover in the mastoid antrum, and forwards, to form 
the roof of the canal for the tensor tympani muscle. It separates the tympanum 
and antrum from the cranial cavity and may contain a few air-cells, whilst occasion- 
ally it is partly deficient. In the child, its outer edge corresponds with the petro- 
il 
