808 



DISSECTION OF THE EAR. 



A thin part 

 of the 

 membrane 

 in notch. 



Eustachian 

 tube: 



part, 

 situation 



and termi- 

 nation ; 



cartilagi- 

 nous part. 



Mastoid 

 cells : 



position and 

 extent ; 



open into 

 mastoid 

 antrum ; 



may 



approach 

 surface ; 



develop- 

 ment. 



Membrane 

 in fenestra 

 rotunda : 



construc- 

 tion 



Occupying the notch above-mentioned in the upper part of the 

 osseous margin (notch of Rivinus), there is a small piece of the 

 membrane which is softer and looser than the rest (membrana 

 flaccida), being formed only by lax connective tissue between the 

 skin and the mucous membrane. . 



The EUSTACHIAN TUBE (fig. 295, e) is the channel through 

 which the tympanic cavity communicates with the external air. It 

 is about an inch and a half in length, and is directed forwards and 

 inwards, as well as somewhat downwards, to the pharynx. Like 

 the meatus auditorius, it is partly osseous and partly cartilaginous 

 in texture. 



The osseous part is rather more than half an inch in length, and 

 is narrowest at its anterior end. Its course in the temporal bone is 

 along the angle of union of the squamous 

 and petrous portions, outside the passage 

 for the carotid artery. Anteriorly it 

 ends in a somewhat oval opening, with 

 an irregular margin, which gives attach- 

 ment to the cartilage. 



The cartilaginous part of the tube is 

 nearly an inch in length, and extends 

 from the temporal bone to the interior 

 of the pharynx. 



Through this tube the mucous mem- 

 brane of the drum of the ear is con- 

 tinuous with that of the pharynx ; and 

 through it the mucus escapes. 



The MASTOID CELLS are air-spaces occu- 

 pying the interior of the temporal bone 

 behind the tympanum and the external 

 auditory meatms. They reach downwards 

 into the mastoid process, and upwards for 

 a short distance into the adjoining region 

 of the squamous portion of the bone. In 

 front they communicate with the tym- 

 panum through a chamber named the 

 antrum mastoideum (fig. 296, am). Above the tympanic membrane 

 is a small recess communicating with the mastoid antrum, which is 

 called the mastoid attic. The size and extent of the cells vary 

 greatly in different individuals ; and in some cases they are sepa- 

 rated only by a very thin layer of bone from the exterior of the 

 skull on the one side, and from the lateral sinus on the other. In 

 the infant the mastoid antrum is present, but the cells are not 

 formed ; the latter are developed at, or a little before, the period 

 of puberty. 



The SECONDARY MEMBRANE OF THE TYMPANUM IS placed in the 



fenestra rotunda, and is rather concave towards the tympanum, 

 but convex towards the cochlear passage which it closes. 



It is formed of three strata, like the membrane on the opposite 

 side of the cavity, viz., an external or mucous, derived from the 



FIG. 297. INNER VIEW OF 

 THE MEMBRANA TYM- 

 PANI IN THE F(ETUS, 

 WITH THE MALLEUS 

 ATTACHED. 



a. Membrane of the 

 tympanum. 



b. Malleus. 



c. Band of circular fibres 

 at the circumference of 

 the membrane. 



d. Anterior, and e, pos- 

 terior tympanic artery. 



/. Tympanic bone. 



