Part 225

1. The =antitragicus= consists of a few bundles which are attached to the conchal cartilage behind the junction of its two borders, and are partially blended with the insertion of the parotido-auricularis.

2. The =helicis= is a small muscle attached in a position opposite to the preceding on the anterior border of the conchal cartilage; it extends also into the depression between the two divisions of the border. It is in part continuous with the insertion of the parotido-auricularis.

3. The =verticalis auriculæ= is a thin stratum of muscular and tendinous fibers which extends upward on the posterior surface of the concha from the eminentia conchæ.

=Actions.=—The base of the concha is rounded and rests in a pad of fat (Corpus adiposum auriculæ), so that the movements of the external ear resemble those of a ball-and-socket joint. It is noticeable that movements about the longitudinal and transverse axes are accompanied by rotation. When the ear is vertical or drawn forward (“pricked up”), the opening is usually directed forward; conversely, when the ears are “laid back,” the opening faces backward. These movements evidently result from the coördinated actions of several muscles which are very complex and cannot be discussed in detail here. The scutularis acting as a whole fixes the scutiform cartilage, so that the muscles which arise on the latter act efficiently on the concha. The anterior auricular muscles in general erect the ear and turn the opening forward. The interscutularis concurs in this action, causing adduction and a symmetrical position of the ears; it also acts directly on the conchal cartilage, since the scutulo-auricularis superficialis superior is in reality a conchal insertion of the interscutularis. The scutulo-auricularis superficialis accessorius draws the concha forward and turns the opening outward. The parieto-auricularis adducts the concha and inclines it forward. The cervico-auricularis superficialis is chiefly an adductor of the conchal cartilage, and directs the opening outward. The cervico-auricularis profundus major turns the opening outward and, acting with the parotido-auricularis, inclines the ear toward the poll. The cervico-auricularis profundus minor tends to direct the opening downward and outward. The parotido-auricularis draws the ear downward and backward, and acts with the cervico-auricularis profundus major in “laying back the ears”; the deep auricular muscles concur in these actions. The tragicus shortens the external auditory canal. The actions of the intrinsic muscles are inappreciable.

The =arteries= of the external ear are derived from the anterior auricular branch of the superficial temporal, the posterior or great auricular branch of the internal maxillary, and the anterior branch of the occipital artery.

The =motor nerves= to the muscles come from the auricular and auriculo-palpebral branches of the facial nerve and from the first and second cervical nerves. The sensory nerves are supplied by the superficial temporal branch of the mandibular nerve and the auricular branch of the vagus.

THE MIDDLE EAR

The =middle ear= (Auris media) or =tympanic cavity= (Cavum tympani) is a space in the tympanic and petrous parts of the temporal bone situated between the membrana tympani and the internal ear. It is a pneumatic cavity, which is lined by mucous membrane, and communicates with the pharynx and the guttural pouch by the Eustachian tube. It contains a chain of auditory ossicles by which the vibrations of the membrana tympani are transmitted to the internal ear.

The cavity consists of: (1) A main part or atrium, which lies immediately to the inner side of the membrana tympani; (2) the recessus epitympanicus, situated above the level of the membrane and containing the upper part of the malleus and the greater part of the incus; (3) a relatively large ventral recess in the bulla ossea.

The =external wall= (Paries membranacea) is formed largely by the thin =membrana tympani=, which closes the inner end of the external auditory canal, and thus forms the septum between the external and middle parts of the ear. The membrane is an oval disc, which slopes obliquely downward and inward at an angle of about 30 degrees with the lower wall of the external auditory canal. The circumference is attached in a groove (Sulcus tympanicus) in the thin ring of bone (Annulus tympanicus) which almost completely surrounds it. The handle of the malleus (the outermost of the auditory ossicles) is attached to the inner surface of the membrane and draws the central part inward, producing a slight concavity of the outer surface. The periphery is thickened, forming the annulus fibrosus. The membrana tympani consists of three layers. The external =cutaneous layer= (Stratum cutaneum) is a prolongation of the lining of the external auditory meatus. The middle =fibrous layer= or =membrana propria= includes two chief sets of fibers; the outer stratum (Stratum radiatum) consists of fibers which radiate from the handle of the malleus, while the inner stratum (Stratum circulare) is composed of circular fibers which are best developed peripherally. There are also branched or dendritic fibers in part of the membrane. The internal =mucous layer= (Stratum mucosum) is a part of the general mucous membrane which lines the tympanic cavity.

FIG. 565.—RIGHT PETROUS TEMPORAL BONE OF HORSE, ANTERO-INTERNAL VIEW.

1, External auditory meatus; 2, annulus tympanicus; 3, laminæ; radiating from 2; 4, membrana tympani; 5, malleus; 6, incus; 7, stapes; 8, fenestra cochleæ (s. rotundum); 9, facial canal; 11, 11″, semicircular canals; 11′, vestibule; 12, cochlea. (After Ellenberger, in Leisering’s Atlas) ]

FIG. 566.—RIGHT AUDITORY OSSICLES AND MEMBRANA TYMPANI, ENLARGED AND VIEWED FROM INNER SIDE AND BELOW.

1, Malleus; 2, incus; 2′, 2″, short and long processes of 2; 3, stapes; 4, tensor tympani; 5, ligament attaching long process of malleus; 6, stapedius muscle; 7, fenestra cochleæ (s. rotundum). (After Ellenberger, in Leisering’s Atlas.) ]

The =internal wall= of the tympanic cavity (Paries labyrinthica) separates it from the internal ear; it presents a number of special features. The =promontory= (Promontorium) is a distinct eminence near the center which corresponds to the first coil of the cochlea, and is marked by a faint groove for the superficial petrosal nerve. Above this is the =fenestra vestibuli= (s. ovalis), a reniform opening which is closed by the foot-plate of the stapes and its annular ligament. The =fenestra cochleæ= (s. rotundum) is situated below and behind the preceding; it is an irregularly oval opening and is closed by a thin membrane (Membrana tympani secundaria), which separates the tympanic cavity from the scala tympani of the cochlea.

The =anterior wall= (Paries tubaria) is narrow, and is pierced by the slit-like tympanic opening of the Eustachian tube. Above this and incompletely separated from it by a thin plate of bone is the semicanal for the tensor tympani muscle.

The =superior wall= or =roof= (Paries tegmentalis) is crossed in its inner part by the facial nerve; here the facial canal is more or less deficient ventrally, and the nerve is covered by the mucous membrane of the tympanum.

The =posterior wall= (Paries mastoidea) presents nothing of importance; a tympanic antrum and mastoid cells, such as are found behind the tympanic cavity proper in man and many animals, are not present in the horse.

The =inferior wall= or =floor= (Paries tympanica) is concave and thin. It is crossed by delicate curved ridges, which radiate from the greater part of the annulus tympanicus.

The =auditory ossicles= (Ossicula auditus) form a chain which extends from the outer to the inner wall of the cavity. They are named, from without inward, the =malleus=, the =incus=, the =os lenticulare=, and the =stapes=. The first is attached to the inner surface of the tympanic membrane and the last is fixed in the fenestra vestibuli.

FIG. 567.—AUDITORY OSSICLES AND MEMBRANI TYMPANI, INNER VIEW, ENLARGED.

1, Annulus tympanicus; 2, membrana tympani; 3, malleus; 4, incus; 5, stapes. (After Ellenberger, in Leisering’s Atlas.) ]

FIG. 568.—RIGHT AUDITORY OSSICLES, ENLARGED AS INDICATED BY LINES GIVING THEIR ACTUAL LENGTH.

a, Malleus; 1, head; 2, neck; 3, handle; 4, long process; 5, muscular process. b, Incus; 1, body; 2, short branch; 3, long branch; c, os lenticulare. c, Stapes; 1, head; 2, crus; 3, base; 4, attachment of stapedius. (After Ellenberger, in Leisering’s Atlas.) ]

The =malleus= or hammer, the largest of the ossicles, consists of a head, neck, handle, and two processes. The =head= (Capitulum mallei) is situated in the epitympanic recess. It is smooth and convex above and in front, and presents on its postero-internal aspect a concave facet for articulation with the body of the incus. The =neck= (Collum mallei) is the constricted part below the head; its inner surface is crossed by the chorda tympani. The =handle= (Manubrium mallei) is directed downward, inward, and a little forward from the neck, and is attached along its entire length to the membrana tympani. On its inner surface, near the upper end, there is a slight projection to which the tendon of the tensor tympani muscle is attached. The =long process= (Processus longus) is a pointed spicule which projects forward from the neck toward the petro-tympanic fissure. The =short process= (Processus brevis) is a slight projection of the outer side of the neck, and is attached to the upper part of the membrana tympani.

The =incus= or anvil is situated chiefly in the epitympanic recess. It may be said to resemble a bicuspid tooth with two divergent roots, and consists of a body and two processes. The =body= (Corpus incudis) articulates with the head of the hammer. The =long process= (Crus longum) projects downward from the body and then curves inward; its extremity has attached to it a small nodule of bone, the os =lenticulare=, which articulates with the head of the stapes. The =short process= (Crus breve) projects chiefly backward, and is attached to the wall of the recess by a small ligament.

The =stapes= or stirrup consists of a head, two crura, and a base. The =head= (Capitulum stapedis) is directed outward and articulates with the os lenticulare. The =crura=, anterior and posterior (Crus anterius, posterius), are directed inward from the head, and join the ends of the base. The =base= or foot-plate (Basis stapedis) occupies the fenestra vestibuli, to which it is attached. The space between the crura and the base is closed by a membrane.

The =articulations= and =ligaments= of the auditory ossicles comprise: (1) A diarthrodial joint between the head of the malleus and the body of the incus (Articulatio incudomalleolaris), inclosed by a capsule. (2) An enarthrosis between the os lenticulare and the head of the stapes (Articulatio incudostapedia), also surrounded by a capsule. (3) The base of the stapes is attached to the margin of the fenestra vestibuli by a ring of elastic fibers (Lig. annulare baseos stapedis). (4) Small ligaments attach the head of the malleus and the short crus of the incus to the roof of the epitympanic recess. (5) The axial ligament (of Helmholtz) attaches the neck of the malleus to a small projection (Spina tympanica anterior) above and in front of the annulus tympanicus.