THE EXTERNAL AUDITORY MEATUS. 



"5* 



value of these cavities, however, for thus concentrating and directing 

 sound-waves, is problematical, as obliteration of the external one by 

 displacement of the auricle backwards has no detrimental effect upon 

 hearing (Gray). 



The width of the meatus varies much in different individuals; in 

 rare cases the greater part of the drum-head may be seen by looking 

 directly into the meatus, without any displacement of the cartilaginous 

 structures. These anatomical variations have not been found to produce 

 any appreciable effect upon hearing. Further, the lumen of the tube 

 may be considerably narrowed by putting in small pellets of wax 

 without any noticeable diminution in the intensity with which sounds 

 are heard. 1 



The protective function of the meatus is more important than any 

 function it may perform of reflecting waves of sound upon the membrane. 

 The tube protects the membrane in two ways (1) By its varying 

 curvature and by its length, and (2) by the secretion of cerumen. As 

 regards the first, it is obvious that any foreign body entering the meatus 

 will come first into contact with one wall and then with another, and 

 may thus be arrested. Further, as the meatus is of considerable length 

 (averaging 24 mm.) in proportion to its transverse area, the air in its 

 cavity maintains a fairly uniform temperature. This temperature is 

 only slightly affected even when the external air is very cold, or when 

 strong winds or draughts blow upon the ear. 



By the secretion of cerumen, the entrance of foreign bodies, especially 

 insects, is prevented. Dust and small particles of foreign matter readily 

 stick to the cerumen, and are thus hindered from passing inwards. 



The chemical analysis of ceru- 

 men shows water, fats, and a yellow 

 pigment, the nature of which has 

 notyet been determined. Cerumen 

 has a bitter taste and an aromatic 

 odour. Probably those qualities 

 deter insects from entering the 

 meatus. They are rarely found in 

 the cavity. 



Cerumen is formed, usually in 

 small quantities, by the tubular 

 glands found in the skin lining 

 the walls of the cartilaginous por- 

 tion. At the outer part of the 

 meatus the glands are scattered 

 throughout the whole circumfer- 

 ence of the tube, but on passing 

 inwards we find that those on the 

 inferior and anterior wall rapidly 

 diminish in number, and cease to 

 exist before the osseous meatus is 

 reached. On the posterior and 

 superior wall they are found 

 further along the tube, and a few 

 even exist in the osseous meatus. 

 Now, since the cartilaginous portion of the tube slopes downwards, from within 

 outwards, it is obvious that the force of gravity will tend to draw the cerumen 



FIG. 418. Posterior wall of the cartilaginous and 

 osseous meatus. a, Orifices of glands on the 

 cartilaginous portion ; b, boundary between 

 osseous and cartilaginous meatus ; c, termina- 

 tion of the triangular space occupied by the 

 orifices of the glands. Politzer. 



1 Politzer, op. cit., p. 57 ; and Arch.f. Ohrenh., Bd. ix. S. 72. 



