IX] THE ORGANS OF SPECIAL SENSE 109 



The p.xtenial ear is made up of the pinna and the external 

 auditory meatus. The pinna, which is varied in shape, is composed 

 chiefly of elastic fibro-cartilage, invested with a thin closely 

 adherent skin. In the domestic animals muscles are present which 

 enable the animal to move the pinnae in the direction of the source 

 of sound. The pinnae by their shape serve to collect and intensify 

 the sound waves transmitted through the air izi the direction of 

 the animal. From the central hollow or concha of the jiinna jiasses 

 a canal, the external auditory meatus, along which the sound « aves 

 pass. The external auditor}^ meatus consists partly of cartilage 

 and parti}" of bone and at its termmation in the bony part of the 

 skull is bounded by a parchment-like membrane called the tym- 

 panic membrane, or drum of the ear. 



The middle ear consists essentially of a ca\4ty in the tem- 

 poral bone of the skull, is lined for the most part Avith ciliated 

 mucous membrane, and communicates mth the cax^itj' of the 

 mouth b}- means of the cylindriform canal called the Eustachian 

 tube, the external opening of which occurs in the pharvTix. The 

 function of the Eustachian tube is to allow the passage of air from 

 the exterior to the middle ear, thus allowing for variations of 

 pressure of the atmosphere and ensuring equality of pressure on 

 both sides of the tympanic membrane. 



The walls of the middle ear are bony except where interrupted 

 by small apertiu-es covered with membrane, as at the fenestra 

 rotunda and fenestra ovalis, and in the outer j)art, the tympanic 

 membrane. The tympanic membrane is tightly stretched in an 

 oblique direction across the end of the external auditory meatus, 

 forming a division between this canal and the middle ear. Stretch- 

 ing from the tympanic membrane across the cavit}" of the middle 

 ear to the fenestra ovalis is a chain of three small bones, the 

 auditory ossicles, called by reason of their shapes the malleus, 

 incus and stapes. The malleus is attached to the tymj^anic mem- 

 brane and the foot of the stapes (or stirrup bone) is attached to 

 the fenestra ovalis, the incus articulating between these two bones. 



The tym^janic membrane and the fenestra ovalis are also 

 provided with small but relatively strong muscles which by their 

 action vary the tension of these two membranes as required. 



The internal ear, or ear proper, consists of a membranous bag 

 (the membranous labyrinth) filled with a fluid called the endo- 

 l3'mph, and this bag is lodged in ca\ities of the petrosal portion 



