THE EUSTACHIAN TUBE. 747 



The anterior extremity of the tympanum is narrowed by the gradual 

 descent of the roof, and is continued into the Eustachiau orifice. The 

 lower compartment of this orifice, lined with mucous membrane, forms the 

 commencement of the Eustachian tube ; the upper compartment, about 

 half an inch long, lodges the tensor tympaui muscle, and opens into the 

 tympanum immediately in front of the feuestra ovalis, surrounded by the 

 expanded and everted end of the cochleariforrn process, which separates it 

 from the lower compartment. 



Fig. 502. 



Fig. 502. ANTERO-POSTEKIOR SECTION OP THK TEMPORAL BONE, SHOWING THE INNER 

 WALL OF THE TYMPANUM, wna THE EUSTACHIAN TUBE AND SMALL BONES IN THE 

 RECENT STATE (from Arnold). 



1, styloid process ; 2, mastoid process ; 3, tipper part of the petrous bone ; 4, pharyn- 

 geal end of the Eustachian tube ; 5, its cartilage ; 6, its mucous surface ; 7, carotid 

 caual ; 8, fenestra rotunda; 9, malleus; 10, incus; 11, stapes; 12, pyramid and 

 stapedius muscle ; above 9, and behind 10, the suspensory ligaments of the malleus a ;d 

 incus are also seen. 



The Eustachian tube is a canal, formed partly of bone, partly of cartilage 

 and membrane, which leads from the cavity of the tympanum to the upper 

 part of the pharynx. From the tympanum it is directed forwards and 

 inwards, with a little inclination downwards ; and its entire length is about 

 an inch and a half. The osseous division of the Eustachian tube, already 

 described in the Osteology, is placed in the angle of junction of the petrous 

 portion of the temporal bone with the squamous portion. The anterior 

 part of the tube is formed of a triangular piece of cartilage, the edges of 

 which are slightly curled round towards each other, leaving an interval at 

 the under side, in which the canal is completed by dense but pliable fibrous 

 membrane. Narrow behind, the tube gradually expands till it becomes 

 wide and trumpet-shaped in front ; and the anterior part is compressed 

 from side to side, and is fixed to the inner pterygoid process of the sphenoid 

 bone. The anterior opening is oval in form, and is placed obliquely at 

 the side and upper part of the pharynx, into which its prominent margin 

 projects behind the lower meatus of the nose, and above the level of the 

 hard palate. Through this aperture the mucous membrane of the pharynx 

 is continuous with that which lines the tympanum, and under certain, 

 conditions air passes into and out of that cavity. 



