The external ear consists of the expanded portion named the auricula or pinna, and the external acoustic meatus. The former projects from the side of the head and serves to collect the vibrations of the air by which sound is produced; the latter leads inward from the bottom of the auricula and conducts the vibrations to the tympanic cavity.
The Auricula or Pinna is of an ovoid form, with its larger end directed upward. Its lateral surface is irregularly concave, directed slightly forward, and presents numerous eminences and depressions to which names have been assigned. The prominent rim of the auricula is called the helix; where the helix turns downward behind, a small tubercle, the auricular tubercle of Darwin, is frequently seen; this tubercle is very evident about the sixth month of fetal life when the whole auricula has a close resemblance to that of some of the adult monkeys. Another curved prominence, parallel with and in front of the helix, is called the antihelix; this divides above into two crura, between which is a triangular depression, the fossa triangularis. The narrow curved depression between the helix and the antihelix is called the scapha; the antihelix describes a curve around a deep, capacious cavity, the concha, which is partially divided into two parts by the crush or commencement of the helix; the upper part is termed the cymba conchæ, the lower part the cavum conchæ. In front of the concha, and projecting backward over the meatus, is a small pointed eminence, the tragus, so called from its being generally covered on its under surface with a tuft of hair, resembling a goat’s beard. Opposite the tragus, and separated from it by the intertragic notch, is a small tubercle, the antitragus. Below this is the lobule, composed of tough areolar and adipose tissues, and wanting the firmness and elasticity of the rest of the auricula.
It is oval in shape, measuring 9 X 10 mm. It is placed obliquely at an angle of 55 degrees with the floor of the meatus. it faces downwards, forwards and laterally.
The membrane has outer and inner surfaces.
The outer surface of the membrane is lined by thin skin. It is concave.
the inner surface is convex. The point of maximum convexity lies at the tip of the handle of the malleus and is called the umbo.
While the greater part of the tympanic membrane is tightly stretched and is, therefore, called the pars tensa, the part between the two malleolar folds is loose and is called the pars flaccid. The parts flaccid is crossed internally by the chord tympani. This part is more liable to rupture than the pars tensa.
The tympanic membrane is composed of the following three layers:
- The outer cuticular layer of skin.
- The middle fibrous layer made up of superficial radiating fibers and deep circular fibers.
- The inner mucous layer is lined by a low ciliated columnar epithelium.
Outer Surface: The anteroinferior part is supplied by the auriculotemporal nerve and the posterosuperior part b the auricular branch of the vagus nerve with a communicating branch from facial nerve.
Inner surface: This is supplied by the tympanic branch of the glossopharyngeal nerve through the tympanic plexus.