INTERNAL EAR 209 



as not less than l - 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 modiolus 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 ex- 

 tending from the modiolus about f 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 membrane 

 of the fenestra rotunda does similar service to the lower duct, the 

 scala tympani. 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. 45). 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 membrane, 

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

 separated from the outer wall of the ojsseous 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 tympani 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 communication exists 

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

 is a narrow tube, the canalis reuniens, which runs from this duct 

 to the saccule part of the organ for maintaining equilibrium. 



The reason for the attention that has been directed to the scala 



B.B. 14 



