EXTEENAL ACOUSTIC MEATUS. 



831 



The lumen of the pars cartilaginea is influenced by the movements of the 

 mandible, being increased when that bone is depressed. This can be easily verified 

 by inserting a 

 finger into 

 nieatus, and 



Pars ossea of external acoustic meatus 



Recessus 



epitympanicus 

 Malleus 

 Cochlea 



Cavum tympani 



Membrana 



tympani 



Internal carotid 

 artery 



Crus antihelicis inferior 

 Cymba conchse 

 Cms helicis 



Pars cartilaginea of 

 external acoustic meatus 

 Cavum conchse 



Lower boundary of 

 incisura intertragica 



FIG. 707. FRONTAL SECTION OF RIGHT EAR ; ANTERIOR HALF OF SECTION, 

 viewed from behind (natural size). 



Umbo 



the 

 then 



alternately opening 



and shutting the 



mouth. 



The condyle of 



the mandible lies 



in front of the pars 



ossea, while between 



the condyle and the 



pars cartilaginea 



a portion of the 



parotid gland is 



sometimes present. 



Behind the pars 



ossea, and separated 



from it by a thin 



plate of bone, are the 



mastoid air-cells. 



Structure of the Meatus. The cartilage of the meatus, directly continuous 



with that of the auricula, is folded on itself to form a groove, opening upwards 



and backwards, the margins of which are connected by fibrous tissue. The medial 



end of the cartilaginous tube is firmly fixed to the lateral margin of the bony 



meatus, whilst its lateral extremity is continuous with the cartilage of the tragus 



(p. 829). Two fissures 

 exist in the anterior 

 portion of the pars 

 cartilaginea, and are 

 filled by fibrous tissue. 



.Membrana tympani In the lateral part of 



11 of the meatus the cartil- 



age forms about three- 

 fourths of the circum- 

 ference of the tube ; 

 but, near the medial 

 end of the pars cartil- 

 aginea the cartilage 

 forms merely a part 

 of the anterior and 

 lower boundaries of 

 the canal. 



IG. 708. HORIZONTAL SECTION THROUGH RIGHT EAR; UPPER HALF OF 



SECTION, seen from below (natural size). The pars ossea of the 



meatus is described on p. 



127 ; but it may be well to state here that in the new-born child it is represented only by an 

 incomplete ring of bone, the annulus tympanicus, together with a small portion of the squama 

 temporalis, which articulates with, and bridges over the interval between, the extremities of the 

 ring superiorly. In the concavity of the annulus is a groove, the sulcus tympanicus, in which the 

 circumference' of the membrana tympani is fixed. On the medial surface of the anterior part of 

 the annulus, a little below its free extremity, a groove, the sulcus malleolaris, is directed down- 

 wards and forwards. It transmits the anterior process and the anterior ligament of the malleus, 

 the tympanic artery, and the chorda tympani nerve. It is limited above by a ridge, the 

 crista spinarum (Henle), which ends in front and behind in a spinous process (spina tympanica 

 major and minor). Below the sulcus malleolaris there is a second, less prominent ridge, 

 the crista tympanica (Gruber), which subsequently unites with a process of the tegmen tympani, 

 so shuts off the canalis musculotubarius from the petrotympanic fissure. A fibrous 

 tympanic plate (Symington) intervenes between the annulus tympanicus and the cartilage 

 of the meatus, and into this plate the bony ring extends. The bony outgrowth does not, 

 however, proceed uniformly from the whole of circumference of the annulus, but occurs most 

 rapidly in its anterior and posterior parts. These outgrowths fuse about the end of the second 



Dndylo of 

 mandible 



Dtid gland 



Tragus 



)ncha 



tihelix 



Helix 



um tympani 



Mastoid air-cells 



Transverse sinus 



