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A TEXTBOOK OF PHYSIOLOGY 



hearing powers. In cetacea there are 1J, in man 2, and in the 

 rodent Coelogenys paca 5 whorls to the cochlea. 



The External Ear. In mammals this varies greatly in form, from 

 the rigid, almost immovable structure of man to the easily movable 

 organ of most mammals, which in some may be rigid, in others flapping. 

 The external ear consists essentially of two parts the auricle, which 

 acts as a sound-catcher and reflector; and the external meatus, by 

 which the sound-waves are conducted down to the drum-head, or 

 tympanic membrane. The external ear is also protective in function. 

 The bitter waxy secretion, or cerumen, and the outward pointing hairs, 

 deter insects from entering, while the varying curvature makes it 

 difficult for foreign bodies directly to impinge upon the tympanic 

 membrane. 



The tympanic membrane separates the external from the middle 

 ear. It is firmly fixed in a bony groove, and lies obliquely to the 

 / H lumen of the meatus, the lower 



margin being farther in than the 

 upper. The membrane consists 

 mainly of connective tissue, to- 

 gether with a little elastic tissue. 

 Some of the fibres radiate to the 

 circumference from the umbo, a 

 point just below the centre of the 

 membrane, the others being 

 arranged circularly about the 

 same point. Into the membrane 

 the first of the three bones of the 

 middle ear is inserted in such a 

 way as to render the membrane 

 conical in shape, with the con- 

 vexity towards the meatus. The 

 membrane thus curves slightly 

 outwards, and is not uniformly 

 stretched in all dimensions. The 

 value of this arrangement is that 

 very slight changes of air-pressure 

 produce relatively large move- 

 ments of the membrane, and 

 It also enables the membrane 



FIG. 373. DIAGRAM OF EAK, SHOWING 

 OSSICLE?. 



.4, Malleus; B, incus; C, stapes; D, ex- 

 ternal auditory meatus; E, tympanic 

 membrane ; F , foramen rotund um ; H , 

 Eustachian tubs ; K, utricle ; L, saccule ; 

 JI, semicircular canals; A T , cochlea. 

 The shaded part of internal ear, the 

 bony labyrinth, is full of perilymph; 

 the white part, the membranous laby- 

 rinth, is full of endolymph. 



therefore relatively great effects, 

 to vibrate to a great range of tones. 



The condition of the membrane is examined by the use of a specu- 

 lum and the reflected light from a mirror with a central hole, attached 

 to the forehead of the observer (Fig. 374). 



Across the middle ear stretches a chain of three small bones, or 

 ossicles the malleus, or hammer ; the incus, or anvil ; and the stapes, 

 or stirrup. The stapes is inserted by ligamentous tissue into the 

 fenestra ovalis. 



The function of the ossicles is to interpose a solid element which 



