256 OUTPOSTS OF THE INTELLIGENCE SERVICE 



the tympanic membrane is about twenty times the area of the 

 fenestra ovalis. This means that, keeping the total power constant, 

 the power per unit area is increased twenty times. This is 

 augmented by the intermediate leverage (correcting for air- 

 damping, friction, etc), which we have seen has been estimated 

 as not less than 1^ — 1. This would give a total increase of effective 

 pressure of at least 30 — 1. (Wrightson puts the value as high as 

 60 — 1 on the assumption that no slip occurs at the malleo-incal 

 joint, etc.) 



3. Internal ear. The internal ear is a somewhat complex 

 cavity in the petrous part of the temporal bone. Two separate 

 organs are housed in this cavity, viz. the labyrinth by which 

 equilibrium is maintained, and the cochlea. 



The Cochlea is a tube, 20-30 mm. long, which takes two and a 

 half spiral turns round a conical bone, the modiohis through the 

 centre of which the auditory nerve passes. The cochlea is divided 

 into three portions by means of (a) a spiral lamina of bone extend- 

 ing from the modiolus about two-thirds across the tube, and 

 (b) joined to the walls of the tube by two membranes, Reissner's 

 and the basilar membrane. The former is a thin layer of cells 

 and separates the vestibular duct from the intramembranous 

 middle duct. The part below the basilar membrane is called the 

 tympanic duct. The fenestra ovalis closes the vestibule — the 

 swelling at the wide end of the scala vestibuli — while the fenestra 

 rotunda does similar service to the lower duct, the scala tijmpani. 

 The two ducts are united at the apex of the cochlea by an irregular 

 crescentic aperture called the helicotrema. This opening has an 

 average area of 0'15 sq. mm. — markedly less than the sectional 

 area of the terminal part of either scala. These scalae are filled 

 with a fluid, perilymph, which obviously, because of the fenestra 

 rotunda, is normally under atmospheric pressure. 



Chief interest in the internal ear lies in the structure of the 

 scala media and its contents (Fig. 65). It is triangular in section, 

 having for base the basilar membrane which separates it from the 

 tympanic duct ; the long side is composed of Reissner''s ynembrane, 

 which divides it from the vestibular duct. The short side is 

 separated from the outer wall of the osseous cochlea by a vascular 

 layer {stria vascularis), laid on and in a continuation of Reissner's 

 membrane, which in turn is placed on the spiral ligament, or pad. 

 Roughly, the cubic capacity of this duct is about a quarter of 

 that of the scala tjpnpani and about a third of that of the scala 

 vestibuli. It is filled with endolymph, a fluid similar to the peri- 

 lymph of the rest of the cochlea. No conmmnication exists 

 between the scala media and any other part of the cochlea. There 



