170 



VETERINAEY PHYSIOLOGY 



of the tensor palati are attached, so that, when this muscle 

 acts in swallowing, the arch of cartilage is drawn down and 



flattened, and the tube opened up 

 (i^g, 84). The Eustachian tube may 

 sret occluded, as a result of catarrh of 

 the pharynx, and the oxygen in the 

 middle ear is then absorbed by the tissues, 

 and the pressure falls. As a result, the 

 membrane is driven inwards by the 

 atmospheric pressure, and does not 

 readily vibrate, and hearing is impaired. 

 When this blocking occurs, and it is 

 necessary to force air into the middle 

 ear, a tube connected with a rubber 

 ball is inserted into a nostril, the 

 mouth and other nostril are closed, 

 and, as the patient swallows, air 

 is forced into the naso - pharynx and 



Fig. 84.— Trans- 

 verse Section 

 through the car- 

 tilaginous lower 

 part of Eustachian 

 Tube, to show the 

 cartilaginous arch 

 cut across, and the 

 way in which it is 

 pulled down and 

 the tube opened 

 in swallowing 

 (shaded). 



so through the Eustachian tube. 



4. Internal Ear. 



The internal ear is a somewhat complex cavity in the 

 petrous part of the temj)oral bone, the osseous labyrinth. It 

 is filled with fluid, the perilymph. It consists of a central 

 space, the vestibule (F.), into which the fenestra ovalis opens. 

 From the anterior part of this, a canal makes two and a half 

 turns round a central pillar. This is the osseous cochlea 

 (fig. 83, Cock). From the central pillar a bony shelf 

 (fig. 85, L.) projects into the canal. From the edge of the 

 bony shelf the basilar membrane extends to the outer wall of the 

 cochlea (fig. 85, B.M.). It is composed of fibres radiating 

 outwards. The inner is more elastic than the outer part. 

 At the base of the cochlea the bony lamella is broad, but at 

 the apex its place is chiefly taken by the membrane, which 

 there measures about three times its width at the base. 



A canal which communicates with the vestibule, into which 

 the oval window opens, called the scala vestibuli (fig. 85, S.V.), 

 thus runs above the bony shelf and basilar membrane, while 

 another canal, the scala tymjxini, runs below them, and ends 



