. Kirkes' handbook of physiology . eous glands are present toward theouter part of the canal, while deeper in the canal are small glands, resemblingthe sweat glands in structure, which secrete the cerumen. Regarding the external ear, therefore, as a collector and conductor ofsonorous vibrations, all its inequalities, elevations, and depressions becomeof evident importance; for those elevations and depressions upon which theundulations fall will tend to intensify certain sound waves while not affectingothers. It is thought that this forms at least an aid in determining the direc-tion whence a s
. Kirkes' handbook of physiology . eous glands are present toward theouter part of the canal, while deeper in the canal are small glands, resemblingthe sweat glands in structure, which secrete the cerumen. Regarding the external ear, therefore, as a collector and conductor ofsonorous vibrations, all its inequalities, elevations, and depressions becomeof evident importance; for those elevations and depressions upon which theundulations fall will tend to intensify certain sound waves while not affectingothers. It is thought that this forms at least an aid in determining the direc-tion whence a sound comes. THE .MlliniJ-: EAR OK M 615 The Middle Ear or Tympanum. The middle ear, or tympanum,3, figure 430, is separated by the membrana tympani from the external auditorycanal. It is a cavity in the temporal bone, opening through its anterior andinner wall into the Eustachian tube. The Eustachian canal establishes communication between the tympaniccavity and pharynx, thus equalizing the air pressure on the sides of the. Fig. 430.—Diagrammatic View from Before of the Parts Composing the Organ of Hearing ofthe Left Side. The temporal bone of the left side, with the accompanying soft parts, has beendetached from the head, and a section has been carried through it transversely, so as to removethe front of the meatus externus, half the tympanic membrane, the upper and anterior wall ofthe tympanum and Eustachian tube. The meatus internus has also been opened, and the bonylabyrinth exposed by the removal of the surrounding parts of the petrous bone. 1, The pinnaand lobe; 2, 2, meatus externus; 2, membrana tympani; 3, cavity of the tympanum; 3, itsopening backward into the mastoid cells; between 3 and 3, the chain of small bones; 4, Eusta-chian tube; s. meatus internus, containing the facial (uppermost) and the auditory nerves; 6,placed on the vestibule of the labyrinth above the fenestra ovalis; a, apex of the petrous bone;b, internal carotid artery; c, styloid process;
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