Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/952

Rh 892 ANATOMY [ORGANS OF SENSE helix is situated where it arises out of the concha ; the muscle of the tragus lies on the front of that prominence ; the mmde of the anti-tragus is placed on the back of that prominence ; the transverse muscle on the posterior or cranial surface of the auricle. The External Meatus leads from the bottom of the concha into the temporal bone, and is separated from the tympanum or middle ear by the membrana tympani. It is a crooked passage one and quarter inch long, inclined at first forwards and upwards, then downwards and inwards. The wall of the outer end of the passage is formed of nbro-cartilage continuous with the cartilage of the auricle, whilst that of the deeper end is formed of the plate-like tympanic part of the temporal bone. The passage is lined with integu ment continuous with the skin of the auricle, in which are situated numerous hairs, together with ceruminous glands which secrete the well-known yellow &quot; wax.&quot; Middle ear. The Tympanum, or Drum, or Middle Ear, is a chamber irregularly cuboidal in form, situated in the temporal bone between the bottom of the meatus and the internal ear. The outer wall is formed of the membrani tympani, which inclines obliquely downwards and inwards at the bottom of the external meatus, at an angle of 55 to the axis of the meatus, whilst the membranes in the two ears form with each other an obtuse angle of 130 to 135. The tym panic membrane is attached to a groove at the bottom of the meatus, and is concave on its outer, convex on its inner surface. It consists of three layers : an external tegumen- tary, continuous with the skin of the meatus, which con tains no hairs or glands ; an internal mucous, continuous with the mucous lining of the tympanum ; and an inter mediate membrana propria, which consists of unyielding fibres arranged both radially and circularly. The radial fibres radiate from the point of attachment of the handle of the malleus. The membrana propria is usually said to be destitute both of nerves and vessels, but Kessel states that nerves, blood, and lymph vessels exist in it as well as in the mucous and tegumentary layers. Immediately in front of the membrana tympani is the Glaserian fissure. The inner wall separates the tympanum from the labyrinth, and presents the following appearances : a rounded elevation or promontory caused by the first turn of the cochlea, on the surface of which promontory are grooves for the lodgment of the tympanic plexus of nerves ; above the promontory is an oval opening closed in by a membrane, the fenestra ovalis, which corresponds with the vestibule ; behind and below the promontory is a round opening closed in by a membrane, the fenestra rotunda, which corresponds with the tympanic passage in the cochlea. The floor of the tympanum is a narrow chink between the inner and outer walls ; and the roof is formed by the anterior surface of the petrous-temporal bone. At its anterior wall the tympanum opens into the Eustachian tube, a canal which communicates with the nasal compartment of the pharynx immediately behind the inferior turbinal. The wall of the tympanic end of this tube is formed of bone, that of the pharyngeal end of a curved plate of hyaline cartilage, which is connected to the bone by nbro-cartilage ; its pharyngeal orifice is dilated into a trumpet-shaped mouth ; through this tube the ciliated mucous membrane of the nasal part of the pharynx is prolonged into the tympanum. The cartilaginous wall of the tube does not completely Biirround it, but is completed by a fibrous membrane, and a layer of voluntary muscle, named by Riidinger the dila tator tubse. Above the tympanic orifice of the Eustachian tube is a fine canal, through which the tensor tympani muscle enters the tympanum. At its posterior wall the tympanum communicates with the air-sinuses in the mastoid temporal ; here also is found a small hollow eminence, the pyramid, through a hole at the apex of which the ten don of the stapedius muscle passea ; and a foramen which transmits the chorda tympani nerve. The tympanic cavity contains three small bones, named malleus, incus, and stapes, arranged so as to form an irregular chain, stretching across the cavity from the outer to the inner wall. The Malleus or hammer is the most external bone. In it may be recognised a head separated by a constricted neck from an elongated handle. Close to the junction of tho neck and handle along slender process projects downwards and forwards to be inserted into the Glaserian fissure, and near the root of the long process a short process projects outwards. By its handle the malleus is intimately con nected with the centre of the membrana tympani ; by its head it articulates with the incus ; whilst ligamentous fibres pass from it upwards, forwards, outwards, and backwards to the tympanic walls. The Incus, or anvil-shaped bone, possesses a body and two processes; on the anterior surface of the body is a saddle-shaped hollow in which the head of the malleus fits; the short process projects almost horizontally backwards, and is attached by a ligament to the posterior wall of the tympanum ; the long process extends at first down wards and then inwards, to end in a rounded projection, named os orbiculare, through which it articulates with tho stapes. The Stapes, or stirrup-shaped bone, possesses a head and neck, a base and two crura ; the head articulates with the os orbiculare of the incus ; from the constricted neck the two crura curve inwards to the base, which is attached to the fenestra ovalis. The joint between the malleus and incus is diarthrodial and saddle-shaped, and the articular surfaces are enclosed by a capsular ligament. The joint between the incus and stapes is also diarthrodial, and pos sesses an investing capsular ligament. Toynbee and Rud inger have described the base of the stapes and the margin of the fenestra ovalis as each invested by hyaline cartilage. Between these plates elastic fibres extend in a plexiform manner, and the intervals between them are occupied by fluid ; the joint seems, therefore, a modified amphiarthrosis. The bones are moved on each other at these joints by small muscles. The tensor tympani arises from the apex of the petrous temporal, and the cartilage of the Eustachian tube enters the tympanum at its anterior wall, and is inserted into the malleus near the root. The laxator tympani muscle arises from the spine of the sphenoid, and the car tilage of the Eustachian tube enters the tympanum through the Glaserian fissure, and is inserted into the neck of tho malleus. The stapedius arises within the pyramid, enters the tympanum through the hole at its apex, and is inserted into the neck of the stapes. The tympanum is lined by a mucous membrane continuous with that of the Eustachian tube, which invests the tympanic ossicles, ligaments, and muscles, and is prolonged backwards so as to line tho mastoid air-sinuses. The epithelium covering this mem brane, where it Hues the floor and the adjacent part of the anterior, posterior, and internal walls, consists of ciliated columnar cells; but the epithelium covering the roof, the promontory, the membrani tympani, and the tympanic ossicles, is tessellated. In the sub-epithelial connective tissue the blood and lymph vessels and nerves of the tympanum ramify. Kessel has recently described in it certain peculiar bodies, which consist of a central axial band with a series of capsules, possessing a fibrillar struc ture, arranged concentrically around the axis ; the function of these bodies is not known. The formation of the auricle and external meatus is well adapted for collecting and transmitting sound-vibrations inwards to the middle ear and labyrinth. These vibrations strike the membrana tympani, and are propagated by the