The External Acoustic Meatus (meatus acusticus externus; external auditory canal or meatus) extends from the bottom of the concha to the tympanic membrane :
- It is about 4 cm. in length if measured from the tragus; from the bottom of the concha its length is about 2.5 cm.
- It forms an S-shaped curve, and is directed at first inward, forward, and slightly upward (pars externa); it then passes inward and backward (pars media), and lastly is carried inward, forward, and slightly downward (pars interna). It is an oval cylindrical canal, the greatest diameter being directed downward and backward at the external orifice, but nearly horizontally at the inner end.
- It presents two constrictions, one near the inner end of the cartilaginous portion, and another, the isthmus, in the osseous portion, about 2 cm. from the bottom of the concha.
The tympanic membrane, which closes the inner end of the meatus, is obliquely directed; in consequence of this the floor and anterior wall of the meatus are longer than the roof and posterior wall.
The external acoustic meatus is formed partly by cartilage and membrane, and partly by bone, and is lined by skin.
- The cartilaginous portion (meatus acusticus externus cartilagineus) is about 8 mm. in length; it is continuous with the cartilage of the auricula, and firmly attached to the circumference of the auditory process of the temporal bone. The cartilage is deficient at the upper and back part of the meatus, its place being supplied by fibrous membrane; two or three deep fissures are present in the anterior part of the cartilage.
- The osseous portion (meatus acusticus externus osseus) is about 16 mm. in length, and is narrower than the cartilaginous portion. It is directed inward and a little forward, forming in its course a slight curve the convexity of which is upward and backward. Its inner end is smaller than the outer, and sloped, the anterior wall projecting beyond the posterior for about 4 mm.; it is marked, except at its upper part, by a narrow groove, the tympanic sulcus, in which the circumference of the tympanic membrane is attached. Its outer end is dilated and rough in the greater part of its circumference, for the attachment of the cartilage of the auricula. The front and lower parts of the osseous portion are formed by a curved plate of bone, the tympanic part of the temporal, which, in the fetus, exists as a separate ring (annulus tympanicus,) incomplete at its upper part.
- The skin lining the meatus is very thin; adheres closely to the cartilaginous and osseous portions of the tube, and covers the outer surface of the tympanic membrane. After maceration, the thin pouch of epidermis, when withdrawn, preserves the form of the meatus. In the thick subcutaneous tissue of the cartilaginous part of the meatus are numerous ceruminous glands, which secrete the ear-wax; their structure resembles that of the sudoriferous glands.
Relations of the Meatus.—In front of the osseous part is the condyle of the mandible, which however, is frequently separated from the cartilaginous part by a portion of the parotid gland. The movements of the jaw influence to some extent the lumen of this latter portion. Behind the osseous part are the mastoid air cells, separated from the meatus by a thin layer of bone.
The arteries supplying the meatus are branches from the posterior auricular, internal maxillary, and temporal.
The nerves are chiefly derived from the auriculotemporal branch of the mandibular nerve and the auricular branch of the vagus.
This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from http://www.bartleby.com/107/).