Henry Gray (18211865). Anatomy of the Human Body. 1918.
1d. 1. The External Ear
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(Fig. 904) 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 crus 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 goats 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.
Structure.The auricula is composed of a thin plate of yellow fibrocartilage, covered with integument, and connected to the surrounding parts by ligaments and muscles; and to the commencement of the external acoustic meatus by fibrous tissue.
The skin is thin, closely adherent to the cartilage, and covered with fine hairs furnished with sebaceous glands, which are most numerous in the concha and scaphoid fossa. On the tragus and antitragus the hairs are strong and numerous. The skin of the auricula is continuous with that lining the external acoustic meatus.
The cartilage of the auricula (cartilago auriculæ; cartilage of the pinna) (Figs. 905,906) consists of a single piece; it gives form to this part of the ear, and upon its surface are found the eminences and depressions above described. It is absent from the lobule; it is deficient, also, between the tragus and beginning of the helix, the gap being filled up by dense fibrous tissue. At the front part of the auricula, where the helix bends upward, is a small projection of cartilage, called the spina helicis, while in the lower part of the helix the cartilage is prolonged downward as a tail-like process, the cauda helicis; this is separated from the antihelix by a fissure, the fissura antitragohelicina. The cranial aspect of the cartilage exhibits a transverse furrow, the sulcus antihelicis transversus, which corresponds with the inferior crus of the antihelix and separates the eminentia conchæ from the eminentia triangularis. The eminentia conchæ is crossed by a vertical ridge (ponticulus), which gives attachment to the Auricularis posterior muscle. In the cartilage of the auricula are two fissures, one behind the crus helicis and another in the tragus.
The ligaments of the auricula (ligamenti auricularia [Valsalva]; ligaments of the pinna) consist of two sets: (1) extrinsic, connecting it to the side of the head; (2) intrinsic, connecting various parts of its cartilage together.
The extrinsic ligaments are two in number, anterior and posterior. The anterior ligament extends from the tragus and spina helicis to the root of the zygomatic process of the temporal bone. The posterior ligament passes from the posterior surface of the concha to the outer surface of the mastoid process.
The chief intrinsic ligaments are: (a) a strong fibrous band, stretching from the tragus to the commencement of the helix, completing the meatus in front, and partly encircling the boundary of the concha; and (b) a band between the antihelix and the cauda helicis. Other less important bands are found on the cranial surface of the pinna.
The muscles of the auricula(Fig. 906) consist of two sets: (1) the extrinsic, which connect it with the skull and scalp and move the auricula as a whole; and (2) the intrinsic, which extend from one part of the auricle to another.
The Auricularis anterior (Attrahens aurem), the smallest of the three, is thin, fan-shaped, and its fibers are pale and indistinct. It arises from the lateral edge of the galea aponeurotica, and its fibers converge to be inserted into a projection on the front of the helix.
The Auricularis superior (Attolens aurem), the largest of the three, is thin and fan-shaped. Its fibers arise from the galea aponeurotica, and converge to be inserted by a thin, flattened tendon into the upper part of the cranial surface of the auricula.
The Auricularis posterior (Retrahens aurem) consists of two or three fleshy fasciculi, which arise from the mastoid portion of the temporal bone by short aponeurotic fibers. They are inserted into the lower part of the cranial surface of the concha.
Actions.In man, these muscles possess very little action: the Auricularis anterior draws the auricula forward and upward; the Auricularis superior slightly raises it; and the Auricularis posterior draws it backward.
The Transversus auriculæ is placed on the cranial surface of the pinna. It consists of scattered fibers, partly tendinous and partly muscular, extending from the eminentia conchæ to the prominence corresponding with the scapha.
Nerves.The Auriculares anterior and superior and the intrinsic muscles on the lateral surface are supplied by the temporal branch of the facial nerve, the Auricularis posterior and the intrinsic muscles on the cranial surface by the posterior auricular branch of the same nerve.
The sensory nerves are: the great auricular, from the cervical plexus; the auricular branch of the vagus; the auriculotemporal branch of the mandibular nerve; and the lesser occipital from the cervical plexus.
FIG. 907 External and middle ear, opened from the front. Right side. (See enlarged image)
The External Acoustic Meatus (meatus acusticus externus; external auditory canal or meatus) extends from the bottom of the concha to the tympanic membrane (Figs. 907,908). 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 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 (page 146).
FIG. 908 Horizontal section through left ear; upper half of section. (See enlarged image)
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.