THE MIDDLE EAR. 855 



The Eustachian tube is the channel through which the tympanum communicates 

 with the pharynx. Its length is an inch and a half (36 mm.), and its direction 

 downward, forward, and inward, forming an angle of about 45 with the sagittal 

 plane and one of from 30 to 40 with the horizontal plane. It is formed partly 

 of bone, partly of cartilage and fibrous tissue. 



The osseous portion is about half an inch in length. It commences in the 

 anterior wall of the tympanum, below the processus cochleariformis, and, gradually 

 narrowing, terminates at the angle of junction of the petrous and squamous por- 

 tions, its extremity presenting a jagged margin which serves for the attachment of 

 the cartilaginous portion. 



The cartilaginous portion, about an inch in length, is formed of a triangular plate 

 of elastic fibro-cartilage, the apex of which is attached to the margin of the inner 

 extremity of the osseous canal, while its base lies directly under the mucous mem- 

 brane of the naso-pharynx, where it forms an elevation or cushion behind the 

 pharyngeal orifice of the tube. The upper edge of the cartilage is curled upon 

 itself, being bent outw y ard so as to present on transverse section the appearance of 

 a hook ; a groove or furrow is thus produced, which opens below and externally, 

 and this part of the canal is completed by fibrous membrane. The cartilage is fixed 

 to the base of the skull, and lies in a groove between the petrous-temporal and the 

 greater wing of the sphenoid; this groove ends opposite the middle of the internal 

 pterygoid plate. The cartilaginous and bony portions of the tube are not in the 

 same plane, the former inclining downward a little more than the latter. The 

 diameter of the canal is not uniform throughout, being greatest at the pharyngeal 

 orifice and least at the junction of the bony and cartilaginous portions, where it is 

 named the isthmus ; it again expands somewhat as it approaches the tympanic 

 cavity. The position and relations of the pharyngeal orifice are described with 

 the anatomy of the naso-pharynx. Through this canal the mucous membrane of 

 the pharynx is continuous with that which lines the tympanum. The mucous mem- 

 brane is covered with ciliated epithelium and is thin in the osseous portion, while 

 in the cartilaginous portion it contains many mucous glands and near the pharyn- 

 geal orifice a considerable amount of adenoid tissue, which has been named by 

 Gerlach the tube-tonsil. The tube is opened during deglutition by the Salpingo- 

 pharyngeus and Dilator tubre muscles. 



The membrana tympani separates the cavity of the tympanum from the bottom 

 of the external meatus. It is a thin, semi-transparent membrane, nearly oval in 

 form, somewhat broader above than below, and directed very obliquely downward 

 and inward so as to form an angle of about 55 with the floor of the meatus. 

 The greater part of its circumference is thickened to form an annular ring which 

 is fixed in a groove, the sulcus tympanicus, at the inner extremity of the meatus. 

 This sulcus is deficient superiorly at the incisure or notch of Rivinus. From the 

 extremities of this notch two bands, the anterior and posterior malleolar folds, are 

 prolonged to the short process of the malleus. The small, somewhat triangular 

 part of the membrane situated above these folds is lax and thin, and is named the 

 membrana flaccida of Shrapnell ; in it a small orifice is sometimes seen. The 

 handle of the malleus is firmly attached to the inner aspect of the membrane as 

 far as its centre, which it draws inward toward the cavity of the tympanum. The 

 outer surface of the membrane is thus concave, and the most depressed part of 

 this concavity is named the umbo or navel. 



Structure. This membrane is composed of three layers, an external (cuticular), 

 a middle (fibrous), and an internal (mucous). The cuticular lining is derived 

 from the integument lining the meatus. The fibrous layer consists of two strata, 

 an external, of radial fibres, which diverge from the handle of the malleus, and an 

 internal, of circular fibres, which are plentiful around the circumference but sparse 

 and scattered near the centre of the membrane. Branched or dendritic fibres, as 

 pointed out by Grliber, are also present, especially in the posterior half of the 

 membrane. 



The arteries are derived from the deep auricular branch of the internal maxil- 



