A text-book of the diseases of the ear for students and practitioners . ngle of 40° with the horizontal. Its tympanic orifice isabout 2*5 cm. higher than its pharyngeal opening. The length of the canal cannot be exactly determined, as thecommencement of its osseous portion in the tympanic cavity isnot sharply denned ; it measures 34-36 mm., of which the carti-laginous portion forms nearly two-thirds. The very narrow, * Cp. Bischoff jun., Mikroskojrische Analyse der Anastomosen derKopfnerven. Munchen, 1865. THE OSSEOUS PORTION OF THE EUSTACHIAN TUBE 39 slit-like part, the so-called isthmus tuba
A text-book of the diseases of the ear for students and practitioners . ngle of 40° with the horizontal. Its tympanic orifice isabout 2*5 cm. higher than its pharyngeal opening. The length of the canal cannot be exactly determined, as thecommencement of its osseous portion in the tympanic cavity isnot sharply denned ; it measures 34-36 mm., of which the carti-laginous portion forms nearly two-thirds. The very narrow, * Cp. Bischoff jun., Mikroskojrische Analyse der Anastomosen derKopfnerven. Munchen, 1865. THE OSSEOUS PORTION OF THE EUSTACHIAN TUBE 39 slit-like part, the so-called isthmus tuba, situated in the carti-laginous section in front of its point of union with the osseousportion, measures 3 mm. in height and not more than J width. Its distance from the pharyngeal orifice averages24-26 mm. 1. The Osseous Portion of the Eustachian Tube (parsossea tubae auditivae). The lateral osseous portion of the tube is a prolongation ofthe anterior portion of the tympanic cavity directed medially;it is bounded above by the tegmen tympani and the canal for. g h i a Fig. 43.—Eustachian Tube and Tympanic Cavity. a, Membrana tympani ; b, Head of the malleus ; c, Lower end of the handle of themalleus ; d, Body of the incus ; e, Short process of the incus ; /, Tensor tympani ;g, Ostium pharyngeum tubse ; h, Isthmus tuba?; i, Ostium tympanicum tubse.(Right ear.) the tensor tympani, below and medially by the carotid boundary between the tube and the tympanum is notsharply defined, as the superior and lateral walls of the lattermerge without interruption into the former. The boundary ismore sharply defined inferiorly where the obliquely rising 4^ DISEASES OF THE EAR anterior wall of the tympanum curves towards the inferior wallof the osseous tube (Figs. 43, i, and 44, ot). The tympanicorifice of the Eustachian tube lies above this place at the levelof the upper third of the tympanum ; it is irregularly definedand inconstant in size, the height of which measures 4
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