Part 88

=M. antitragicus= (Fig. 169, 6; Fig. 64, n).--A small muscle about one centimeter in length and three millimeters in thickness, attached at one end to the caudal border of the antitragus. The fibres pass mediodistad and are inserted on the tragus, in common with the fibres of the tragicus medialis (Fig. 169, 5, 5′).

Relations.--Outer surface with the adductor auris inferior and the integument; inner surface with the cartilage.

Action.--Constricts the external auditory opening.

=M. tragicus medialis= (Fig. 169, 5, 5′).--A small, flat muscle, in two parts, on the medial surface of the concha.

Origin on the ventral end of the tragus. The cranial fibres form a thick nearly cylindrical bundle which extends proximad about one centimeter, and is inserted on the cranial surface of the concha just proximad of a deep fold in the cartilage. The caudal fibres form a thin sheet which spreads on the medial surface of the concha and is inserted on an obliquely proximodistal line on the medial surface of the concha, less than one centimeter from the free distal edge.

These two bundles are distinct except at their origin, and are sometimes considered as separate muscles.

Relations.--Outer surface with the tragicus lateralis (4) and the fat about the concha. Inner surface with the concha.

Action.--Flexes the concha.

Strauss-Durckheim describes under the name =cornetoconchæus= a small muscle bridging the deep groove caused by the folding of the medial surface of the concha, its proximal end being close to the insertion of the abductor auris brevis (page 100). This muscle was absent in all of the cases examined for its presence.

=M. conchæus externus= (Fig. 169, 9).--A quadrilateral muscle on the lateral surface of the concha. The cranial end of this muscle is attached to the concha a short distance distad of the antitragus along a dorsoventral line six to eight millimeters in length; the fibres pass thence caudodorsad a distance of about one centimeter, where they are again attached to the concha. The muscle thus bridges a shallow depression on the lateral surface of the concha.

Action.--Constricts the concha.

=The Middle Ear.=

The outer wall of the bulla has been removed, carrying with it the tympanic membrane with the malleus and incus; these are now viewed from the side that was turned toward the cavity. a, bony ring forming inner end of external auditory meatus; b, b′, malleus; c, tympanic membrane; d, incus; e, lenticular process; f, chorda tympani nerve; g, cartilaginous support for chorda tympani nerve.]

The middle ear is enclosed in a cavity within the tympanic bulla. Its outer boundary, as well as the inner boundary of the external ear, is formed by the =tympanic membrane= (Fig. 170, c.) This is a thin, semitransparent membrane attached to the bony ring (a) surrounding the inner end of the external auditory meatus. It is oval in outline. Across the dorsal third of its inner surface runs the handle of the malleus (b), to which the membrane is firmly attached and which by pulling on the membrane gives it the form of a cone instead of a flat surface. The apex of the cone is directed mediad.

The middle ear, or =cavum tympani=, is an ellipsoidal cavity situated in the tympanic bulla. On removing the medial wall of the bulla, a bony plate is seen separating its cavity into two chambers. This plate of bone extends in from the lateral wall of the bulla, forming a complete partition except at the dorsocaudal part, where a notch in the bone forms a free communication between the two chambers. The lateral chamber, or tympanum proper, contains the bones of the ear and is limited externally by the tympanic membrane. At its cranial end is seen the opening of the =tuba auditiva=, or =Eustachian tube=.

1, malleus; 2, incus. a, handle of malleus; b, neck; c, head; d, process for attachment of tendon of M. tensor tympani; e, wing-like plate and process of neck; f, g, the two processes of the incus.]

The tuba auditiva, or Eustachian tube, is a cartilaginous tube about one and a half to two centimeters long, passing from the nasopharynx to the middle ear and placing these two cavities in communication. The pharyngeal opening is at about the middle of the length of the nasopharynx, near the dorsal border of the lateral wall; it is a narrow slit two or three millimeters long. Thence the tube passes caudolaterad on the ventral surface of the sphenoid bone, and enters the tympanic cavity through the opening just laterad of the styliform process of the tympanic bulla. The tube has a thick, cartilaginous medial wall, while the lateral wall is thin and formed of connective tissue. The lumen of the tube is a curved slit in cross-section.

=Bones of the Ear= (ossicula auditus).--There are three small bones in the middle ear, the malleus (hammer) (Fig. 171, 1), incus (anvil) (Fig. 171, 2), and stapes (stirrup) (Fig. 172).

The malleus (Fig. 171, 1) consists of a slender handle (a) with a small neck (b) attached at an angle to the caudal end of the handle. At the end of the neck is a rounded head (c) which articulates with the incus (2), and together with one part of the incus lies in a small fossa dorsad of the fenestra vestibuli. From the medial aspect of the neck extends a short process of bone (d) to which is attached the tendon of the tensor tympani muscle. One side of the neck is produced into a thin wing-like plate, bearing on its edge a pointed process (e).

The =incus= (Fig. 171, 2) bears much resemblance to a two-fanged molar tooth. It lies in the fossa with the head of the malleus. Its head or crown is directed craniad and has a concave surface which articulates with the head of the malleus. One of the fangs (f) extends caudad in the fossa above mentioned and is held in position by a ligament. The other fang (g) extends ventromediad and articulates with the head of the stapes. This fang terminates in a minute rounded tubercle known as the =lenticular process=; this in early stages of development is a separate bone, the =os lenticulare=.

a, head; b, base; c, process for attachment of the stapedius muscle.]

The =stapes= (Fig. 172) has the form of a stirrup, and is the last in the chain of bones. The small end is termed the head (a) and the broader part the base (b). The head of the stapes articulates with one of the processes of the incus. The base of the stapes fits into the fenestra vestibuli of the petrous bone and completely closes it. The stapes is held in position by the stapedius muscle, which is attached to a minute process (c) near the head.

Thus the three bones of the middle ear form a chain which connects the medial surface of the tympanic membrane with the fenestra vestibuli. As the fenestra vestibuli forms a portion of the wall of the vestibule, the vibrations of the tympanic membrane are transmitted through the chain of bones to the fluid within the vestibule.

=Muscles of the Middle Ear.=

=M. tensor tympani.=--The tensor tympani muscle is a short conical muscle having origin in a small fossa (Fig. 25, d, p. 34) in the petrous bone dorsocraniad of the fenestra vestibuli. Its very short tendon passes laterad and is inserted into a projection (Fig. 171, d) on the neck of the malleus.

=M. stapedius.=--The stapedius muscle is a short muscle taking origin from a fossa (Fig. 25, f) in the lateral surface of the petrous bone caudad of that for the incus. It is inserted into the head of the stapes.

=The Internal Ear= (Fig. 173).

The internal ear or =labyrinth= is completely enclosed in the petrous bone. In the dry bone the internal ear communicates with the middle ear by the fenestra cochleæ and the fenestra vestibuli, which have been described in the account of the petrous bone (page 33). In the recent state the fenestra vestibuli is closed by the base of the stapes, while the fenestra cochleæ is covered by a delicate membrane,--so that the internal ear forms a closed cavity. It is possible to distinguish a =bony= labyrinth and a =membranous= labyrinth (Fig. 173), the former being the cavity in the petrous bone, while the latter is the real auditory organ, situated within the cavity. The membranous labyrinth repeats in general the form of the bony labyrinth, so that it will not be necessary to distinguish the two in our account of the main features of the structure. The auditory nerve is distributed to the membranous labyrinth. For an account of the minute structure of this organ reference should be made to a text-book of general histology or physiology.