440 NERVOUS SYSTEM. 



with one another. They are distinct from the diploic structure 

 of the bone, as they contain no medulla, and are lined by a 

 continuation of the internal membrane of the tympanum, which 

 is extremely thin upon them. The orifice of communication 

 between these cells and the tympanum, is placed at the supe- 

 rior posterior part of the latter: it is rough and irregular, and 

 partially occupied by the short leg of the incus. Just in front 

 of this opening, the cavity of the tympanum is extended verti- 

 cally, for the purpose of accommodating the body of the mal- 

 leus and of the incus, whereby they arc in a great degree con- 

 cealed, unless the corresponding margin of the tympanum be 

 cut away. 



At the fore part of the tympanum is the Eustachian Tube, 

 which runs for six or eight lines in the substance of the petrous 

 bone, near its exterior margin ; and then terminates in a carti- 

 laginous and membranous portion, which communicates with 

 the pharynx at the posterior naris. The latter extremity of the 

 Eustachian Tube is placed on a line with the posterior end of 

 the inferior spongy bone. Its orifice is rounded or oval, is large 

 enough to admit the tip of the little finger, and reposes against 

 the side of the internal pterygoid process of the sphenoid bone. 

 Though almost within the precincts of the posterior naris, this 

 orifice is to be considered as opening into the pharynx. 



This canal, m its whole length, measures nearly two inches, 

 and, with the exception of the portion in the petrous bone, is 

 cartilaginous and membranous. The cartilage is a single, thick, 

 triangular plate, flat, and adhering by one of its edges to the 

 pterygoid process. The under part of the tube is membranous, 

 thin, and affords attachment to some of the muscles of the soft 

 palate. Its course is nearly horizontal, backward and out- 

 ward. 



It is lined in its whole extent, by a very fine mucous mem- 

 brane continuous with that of the pharynx and of the tympa- 

 num. This membrane is thickened at its anterior extremity by 

 the mucous glands beneath it, which assist in giving the marked 

 elevation to its orifice. The canal diminishes as it goes back- 

 ward, so as to receive with difficulty a small probe. 



Parallel with the bony part of this canal, but above it, and 



