458 EUSTACHIAN TUBE. 



which consists of bone at one extremity, and of cartilage at 

 the other. It commences in the upper and anterior part of 

 the cavity of the tympanum, and continues forwards and 

 inwards through a part of the petrous portion of the temporal 

 bone, above the fissure in the cavity for receiving the condyle 

 of the lower jaw. The bony canal terminates in a very 

 rough and irregular orifice, which is united to a tube com- 

 posed of cartilage and of "membrane, that terminates by a 

 large orifice behind the inferior turbinated bone of the nose, 

 and on a line with it. 



The whole length of the Eustachian Tube is nearly two 

 inches. The bony portion is about eight lines long ; the carti- 

 laginous from an inch to an inch and a quarter. The latter 

 rests against the side of the internal pterygoid process of the 

 sphenoid bone, to the periosteum of which it is attached ; and 

 on its under surface gives attachment to some of the muscles of 

 the palate. The course of the tube is nearly horizontal ; it 

 runs outwards, backwards, and slightly upwards. The open- 

 ing of the tube into the fauces is funnel-shaped and large 

 enough to receive the end of the little finger. It contracts sud- 

 denly, at the place of union of the cartilaginous and bony por- 

 tion, and it is there barely large enough to admit a good sized 

 bristle. From this point the calibre of the tube again increases 

 somewhat in size, in its course to the cavity of the tympanum. 



This tube is lined by a continuation of the membrane of the 

 posterior nares, which becomes more thin as it proceeds 

 towards the cavity of the tympanum. 



Above the Eustachian Tube, and separated from it by a thin 

 plate of bone, is a small canal which is nearly parallel to it, and 



bones has been removed, t, Two openings leading into the cells of the petrous 

 portion of the temporal bone. On the internal wall of the cavity of the tympa- 

 num, are seen two openings, ihefenestra ovalis, leading to the Vestibule, I ; and 

 the fenestra rotunda, leading to the cochlea, n. m, Semicircular canals, k, 

 Eustachian tube leading from the pharynx to the cavity of the tympanum, o, 

 Auditory nerve. The protuberance made on the internal wall of the cavity of 

 the tympanum, is called the promontory. The opening called apertura chordce, 

 and the pyramid, are on the posterior wall of the tympanic cavity, and cannot 

 be well seen in this figure. 



