The =external ear= (Auris externa) comprises (1) the =auricula=, a
funnel-like organ which collects the sound waves, together with its
muscles, and (2) the =external auditory canal=, which conveys these
waves to the tympanic membrane, the partition which separates the canal
from the cavity of the middle ear.
The =auricula= or pinna is attached by its base around the external
auditory canal in such a manner as to be freely movable. In the
following description it will be assumed that the opening is directed
outward and that the long axis is practically vertical. It has two
surfaces, two borders, a base, and an apex. The convex surface or dorsum
(Dorsum auriculæ) faces inward and is widest in its middle part; its
lower part is almost circular in curvature, while above it narrows and
flattens. The concave surface (Scapha) is the reverse of the dorsum; it
presents several ridges which subside toward the apex. The anterior
border is sinuous; it is largely convex, but becomes concave near the
apex. It divides below into two diverging parts (crura helicis). The
posterior border is convex. The apex is flattened, pointed, and curved a
little forward. The base is strongly convex. It is attached to the
external auditory process of the petrous temporal bone, and around this
there is a quantity of fat. The parotid gland overlaps it below and
externally. The structure of the external ear comprises a framework of
cartilages (which are chiefly elastic), the integument, and a
complicated arrangement of muscles.
The =conchal= or =auricular cartilage= (Cartilago auriculæ) determines
the shape of the ear; its form can be made out without dissection,
except below, where it is concealed by the muscles and the parotid
gland. The basal part is coiled to form a tube, which incloses the
cavity of the concha (Cavum conchæ). This part is funnel-shaped and
curves outward and a little backward. Its internal (medial) surface is
strongly convex, forming a prominence termed the =eminentia conchæ=. The
lowest part of the internal margin bears a narrow, pointed prolongation,
the =styloid process=. This process is about an inch long and projects
downward externally over the annular cartilage; the guttural pouch is
attached to its free end. Behind its base there is a foramen through
which the auricular branch of the vagus passes.
The basal part of the posterior border is cut into by a notch, which
separates two irregular quadrilateral plates. The upper plate (Tragus)
is overlapped by the anterior border, and is separated from the
adjacent part of the posterior border (Antitragus) by a notch
(Incisura intertragica). The lower plate is curved to form a half ring
and partly overlaps the anterior border and the annular cartilage.
Behind the notch there is a foramen, which transmits the internal
auricular artery and internal auricular branch of the facial nerve.
[Illustration:
FIG. 562.—CONCHAL AND ANNULAR CARTILAGES OF EAR OF HORSE, EXTERNAL
VIEW.
_1_, Base of concha; _2_, posterior border, _3_, anterior border of
concha; _4_, intertragic notch; _5_, eminentia conchæ; _6_, styloid
process; _7_, annular cartilage; _9_, zygomatic arch. (After
Ellenberger-Baum, Anat. für Künstler.)
]
The =annular cartilage= (Cartilago annularis) is a quadrilateral plate,
curved to form about three-fourths of a ring; its ends are a little less
than half an inch (ca. 1 cm.) apart internally and are united by elastic
tissue. It embraces the external auditory process and forms with the
lower part of the conchal cartilage the cartilaginous part of the
external auditory canal.
The =scutiform cartilage= (Cartilago scutiformis s. scutulum) is an
irregular quadrilateral plate which lies on the temporal muscle in front
of the base of the conchal cartilage. Its superficial face is slightly
convex from side to side and its deep face is correspondingly concave.
The anterior end is thin and rounded; the posterior part or base is
wider and thicker, and its inner angle is prolonged by a pointed process
half an inch or more in length. The cartilage moves very freely over the
underlying parts.
The =external auditory canal= or =meatus= leads from the cavum conchæ to
the tympanic membrane. It does not continue the general direction of the
cavity of the concha, but extends inward, downward, and slightly
forward. It consists of a cartilaginous part formed by the lower part of
the conchal cartilage and the annular cartilage, and an osseous part
formed by the external auditory process of the temporal bone. These are
united by elastic membranes to form a complete tube. Its caliber
diminishes from without inward, so that the lumen of the inner end is
about half of that of the outer end.
The =skin= on the convex surface of the concha presents no special
features; it is attached to the cartilage by a considerable amount of
subcutaneous tissue except at the apex. The integument which lines the
concave surface is intimately adherent to the cartilage and is
relatively dark in color. There are three or four cutaneous ridges which
run about parallel with the borders of the conchal cartilage, but do not
extend to the apex or the cavum conchæ. The upper and marginal parts and
the ridges are covered with long hairs, but between the ridges and below
it is thin, covered sparsely with very fine hairs, and supplied with
numerous sebaceous glands. In the external auditory canal the skin
becomes thinner; in the cartilaginous part it is supplied with numerous
large, coiled, =ceruminous glands= (Glandulæ ceruminosæ) and is sparsely
covered with very fine hairs; in the osseous part the glands are small
and few or absent and there are no hairs.