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Home , Middle ear, Ossicles, Tympanic cavity

CHAPTER 2. STRUCTURE OF THE MIDDLE IN

The structure of mammals has been attracting the attention of various scientists for a long time, so at present there are numerous funda- mental works devoted to a comparative and anatomical study of middle ear elements in the representatives of different orders [Hyrtl, 1844, 1845, 1848; Claudius, 1867; Doran, 1878; Denker, 1899; Bondy, 1907; Frey, 1911; Crowe et al., 1931; Wassif, 1948, 1950; Yamada, 1953 a, b; Webster, 1960, 1966; Werner, 1960; Kobrak, 1963; Hentzen, 1970; Solntseva, 1972b, 1973b, d, f]. Based on the available literature and his own data, German researcher G. Fleischer [1973 a, b] provided a comparative anatomical study of the tym- panic bulla in representatives of different orders. Fleischer’s data, along with the evidence obtained by other researchers mentioned above as well as my- self [Solntseva, 1975a, b], form the basis of this study. The bulla tympanica of mammals represents a periotic-tympanic com- plex (Fig. 8a-i). The tympanic is composed of thin osseous walls, form- ing the tympanum, in which the elements of the middle ear are localized. The is situated in periotic bone. The middle ear of mammals contains a tympanum (cavum tympani), which by means of an auditory tube is connected with the gullet cavity, tympanic mem- brane and a chain of the auditory (ossicula auditus): , and . Ligaments and two muscles of the middle ear (m. tensor tympani, m. stapedius) are bound to the auditory ossicles. Muscles of the tympanum regulate the transition of auditory energy and preserve the inner ear from su- per-intensive sounds. Ligaments keep auditory ossicles in a definite position. The middle ear is separated from the by a tympanic membrane, stretched onto a tympanic ring (anulus tympanicus) (Fig. 9a-c). The tympanic membrane is formed by three layers. Its basis is composed of radially and circu- larly directed connective-tissue fibers, which grow together with the periosteum of the malleus handle (manubrium mallei), set into the tympanic membrane. From inside, the tympanic membrane is covered by a mucous membrane (stra- tum mucosum), formed by a pavement epithelium; from the surface, the tym- panic membrane is covered by a lacking hair, glands and a papillary layer tegu-

CHAPTER 2. STRUCTURE OF THE MIDDLE EAR IN MAMMALS 45 ment (stratum cutaneum). The tympanic membrane has the shape of a cone, with the top directed into the . A stretched part (pars tensa) and a small, unstretched part (pars flacida), which is more flexible due to the lack of the basis, are discernible on the tympanic membrane (Fig. 10a, b). The tympanum is underlaid by a mucous membrane, in the middle layer of which there are blood vessels which regulate the degree of the stratum’s thickness. The tympanum is located between the external auditory meatus and the inner ear labyrinth, and is separated from these formations by thin membranes. Six walls are discernible in the tympanum: an anterior wall, where the orifice of the is situated; a posterior wall, having a foramen leading into the antrum mastoideum; medial, upper, and lateral walls, formed by the tympanic membrane; and a bottom wall. The medial wall is formed by a labyrinth capsule and separates the middle ear from the inner ear. This wall contains oval and round windows. The upper wall has a form of osseous partition [Kobrak, 1963]. The auditory ossicles of mammals are more complicated than in represen- tatives of other classes of terrestrial . The malleus is differentiated into the head of the malleus (capitulum mallei), neck of the malleus (collum mallei) and handle of the malleus. The head of the malleus bears an articular surface for a junction with the incus. At the medial side of the handle of the malleus, near the neck of the malleus, a small muscular (pr. muscularis) is located, which serves for the fastening of a tensor muscle of the tympanic mem- brane. At the dorsal edge of the handle of the malleus, a short arm (pr. brevis) is fastened; and at the dorsal edge of the neck of the malleus, a long arm (pr. longus), which is reduced in most mammals to a small, sharp pr. gracilis, is connected with the help of a ligament within the tympanum’s wall. In other species, the long arm is large in size and connects rigidly with the wall of the tympanum. The m. tensor tympani is attached to the handle of the malleus, under whose contraction the tympanic membrane is pulled inside and, through a system of auditory ossicles, presses the stapes into an . The malleus is fixed in a definite position with the help of three ligaments: a fore bundle that reaches the fissura Glasseri and gets attached to the pr. anterior of the malleus; a ligament located in the area opposite the neck of the malleus; and an axial ligament representing the axis, around which the malleus rotates. The incus has a body and two components: the long arm (crus longum) and the short arm (crus breve). The long arm is stretched parallel to the handle of the malleus. Its lower end is curved, forming a junction with the stapes with the help of a lenticular arm. The short arm is located in the deep- ening of the osseous tympanum, where it is bound by a ligament.

46 PART I. PERIPHERAL PART OF THE OF MAMMALS IN POSTNATAL ONTOGENY