boundary folds. The portion situated above the short process and bounded by the incisura Rivini and the anterior and posterior boundary folds is known as the flaccid portion, membrana flaccida, or Shrapnell's membrane. This portion is frequently traversed by a fold, higher than the other two, which runs obliquely backward and upward from the short process. Retraction of the drumhead produces a pathologic posterior fold, which passes from the short process downward into the tympanic membrane (Plate 38, Fig. 8). The greater portion of the drumhead lies below the short process and is known as the pars tensa. An imaginary line drawn from the extremity of the handle of the malleus to the lower border of the drumhead, and a horizontal line making a right angle with the axis of the handle of the malleus drawn through the drumhead at the level of the umbo (Fig. 6), divide the membrane into a lesser anterior and greater posterior, or into a greater upper and lesser lower, portion. The anterior half is divided into anterosuperior and anteroinferior quadrants; the posterior into posterosuperior and postero-inferior quadrants; the anterosuperior quadrant is the smallest, the posterosuperior the largest. This division of the drumhead is of great practical usefulness, because the pathologic appearances occurring in various portions of the membrane differ greatly in importance. Thus, the anterosuperior quadrant (Plates 10, 38, 39) corresponds to the region of the tympanic opening of the tube, the canal for the tensor tympani muscles, and the anterior pouch of the drumhead. The antero-inferior

quadrant corresponds to the carotid canal. The posterosuperior quadrant contains the long process of the incus, the stapes, and the articulations of these bones, the fenestra cochleæ, the eminentia pyramidalis, and stapedius muscle, the posterior pouch of the drumhead, the chorda tympani, and the posterior fold (pathologic). The postero-inferior quadrant contains the fenestra cochleæ, the tympanic cells in the floor of the tympanic cavity, and the bulb of the jugular vein (Plate 38, Fig. 3). The

flaccid portion, or Shrapnell's membrane, corresponds to the neck of the malleus and Prussak's space (Plates 3, 20).

Histology. The tympanic membrane consists of three layers: (a) The stratum cutaneum, derived from the integument of the auditory meatus; (b) the stratum fibrosum, derived from the tendinous ring (annulus tendinosus); (c) the stratum mucosum, derived from the mucous membrane lining the tympanic cavity. In the pars flaccida only (a) and (e) are represented (Plates 13, 31).

(a) The stratum cutaneum is thin, except in the prolongation of the strip of skin covering the superficies meatus, which passes behind the handle of the malleus (Plate 31). This portion is somewhat thicker and contains within its loose-meshed cuticular layer the malleolar vein, artery, and nerve. It consists of layers of squamous epithelium, with cylindric cells in the deepest, and horny cells in the most superficial, layer. Owing to the eccentric growth of the horny layer, hemorrhages within the epiderm are sometimes displaced laterally as far as, and even beyond, the tympanic ring. The cutis is thin, devoid of glands or hairs, and contains a few poorly developed papillæ and blood-vessels, which become somewhat more conspicuous in the above-mentioned thickened strip.

(b) The stratum fibrosum is composed of an outer layer of radial and an inner layer of circular connective-tissue fibers, crossing one another at acute angles (stratum radiatum and stratum circulare). It contains but few elastic fibers, and the drumhead is accordingly very inelastic. It often bursts with a loud report and wrinkles easily. The fibers of the tendinous ring are continuous with the periosteum and bone of the auditory meatus and of the tympanic cavity. The stratum radiatum is attached to the tendinous ring and to the lower and middle third of the handle of the malleus (Plate 13); it serves to attach this structure to the drumhead by passing around it and becoming adherent with its perios

teum.

Wherever the anterior surface of the handle of the malleus (Plate 31) comes in contact with the drumhead it is covered with hyaline cartilage. The radial fibers of the drumhead are separated by narrow spaces containing connective-tissue cells (corpuscles of the tympanic membrane). The intersection of the circular with the radial fibers begins at the tendinous ring. The circular fibers decrease in number from the periphery to the handle of the malleus, to the upper two-thirds of which they become attached. At the umbo the radial fibers are so closely packed that an opacity is produced whenever cartilage cells are deposited in this region (yellow spot: Trautmann). The thinnest portion of the drumhead is the point where the intermediate zone fades away into the center. The posterior half of the drumhead is occupied by a branching network of connective tissue that bulges the mucous membrane forward (Gruber's dendritic structure).

(c) The stratum mucosum consists of a layer of connective tissue containing numerous lymphocytes covered by a single layer of cuboidal cells (Plate 13). At the junction of the drumhead with the mucous membrane of the tympanic cavity the epithelial cells increase in height and become ciliated (on the floor of the tympanic cavity). The connective tissue forms papillary elevations, the depressions between the papillæ resembling tubular glands. In the new-born the layers (a) and (e) are more strongly developed than in the adult; hence the drumhead is thicker and more opaque and offers greater resistance to perforation in otitis media.

5. The drumhead forms the outer wall of the tympanum; its bony walls have been described under 3, 7. Its name is derived from its shape, which corresponds to a short cylinder and serves for the reception of the tympanic ossicles and their muscles. The inner wall of the promontory (Plates 1, 5) contains the fenestra vestibuli, looking directly forward and parallel to the plane of the drumhead; it is closed by the base of the stapes. The

head of the stapes lies at a somewhat deeper level than the base; to it is attached the tendon of the stapedius muscle, which passes across the posterior portion of the cavity. The fenestra cochleæ lies on the floor of a depression looking toward the posterior wall of the tympanic cavity, and accordingly forms a right angle with the fenestra vestibuli. The fenestra cochleæ is closed by a layer of connective tissue known as the secondary membrane of Scarpa (membrana tympanica secundaria), the outer surface of which is concave and covered with mucous membrane, its inner surface being covered with endothelium; it may be exposed to view by removing the anterior border of the bone (Plate 14). The floor of the tympanic cavity extends below the lower border of the drumhead, forming the "cellar," or hypotympanic recess (Kretschmann); hence secretions on the floor of the tympanic cavity are not necessarily seen through the membrane. When the bulb of the jugular vein is larger than normal, it encroaches on the posterior half of the drumhead; in such cases the bone is often deficient in places as far as the edge of the fenestra cochleæ, and the wall of the vein is directly covered by mucous membrane. This affords passage for pus, and the vein is in danger of injury in paracentesis. When the condition is present, a bluish, crescentic sheen is seen in the posterior lower portion of the drumhead (Plate 17; Plate 38, Fig. 3). At the level of the tympanic opening of the Eustachian tube the knee of the internal carotid, which lies near the median line, approaches the anterior wall, and is therefore beyond the region of the drumhead (Plate 5). Occasionally the carotid canal is displaced into the tympanic cavity as far as the promontory; hence if there is any defect in the bone, there is danger of injuring the carotid artery in paracentesis (Gruber) (Plate 17).

Corresponding to the "cellar," or hypotympanic recess, which extends below the tympanic cavity, is the attic (Hartmann) above, which receives the head of the malleus and the body of the incus, and is bounded, above,

by the tegmen tympani; externally, by the osseous portion of the roof and the posterior wall of the auditory meatus. Hence the drumhead does not correspond exactly to the boundaries of the tympanic cavity either above or below. This has an important bearing on the question of determining the origin of pus in the middle ear. The ossicles establish a communication (Plate 2, Fig. 7) between the drumhead and the fenestra vestibuli; the malleus is on the drumhead, the stapes is in the fenestra vestibuli, and between the two lies the incus (Plate 3). The malleus presents a head that occupies the attic, a neck behind Shrapnell's membrane, and a handle in the pars tensa. The posterior and lateral aspects of the head present an articular surface in the form of an 8. Its inferolateral face presents a prominence, the tooth process; its posterior face an oblique ridge, the crista

mallei.

Underneath are openings for the passage of bloodvessels. The head is joined to the handle by means of the neck, which presents, in front, a small eminence, the long process, the remains of Meckel's cartilage (processus anterior Folii); the handle begins at the short process (processus lateralis), which is seen distinctly in the drumhead, and presents, in front and behind, a roughened area for the insertion of the tensor tympani muscle. The extremity of the handle is expanded (superficies umbilicalis). The incus presents a body occupying the attic, with a lateral articular surface, in the form of an 8, by which it articulates with the malleus, and a more medianly situated eminence, the tooth process; behind and projecting horizontally is the short process (crus breve), which is firmly attached to the floor of the tympanic recess by the posterior ligament of the incus (Plate 4). The long process arches backward and downward through the tympanic cavity, behind and almost parallel with the handle of the malleus. Its extremity articulates by means of a convex tubercle, the processus lenticularis, with the head of the stapes. The stapes lies within the tympanic cav