^58 THE RABBIT. PHYLUM CHORDATA 



Sound waves are probably collected by the pinna, and certainly 

 pass down the outer ear or external auditory meatus to the 

 tympanum or eardrum. This vibrates, and transmits its vibra- 

 tions to the chain of ear ossicles, the malleus, incus, and stapes, 

 in the middle ear. The last of these pushes in and out against 

 the membrane of the fenestra ovahs, and so causes pressure 

 differences in the lluid called perilymph surrounding the sensitive 



em 



Fig. 367. — Diagram of ear of mammal in vertical section. — From Goodrich, 

 Studies on the Structure and Development of Vertebrates, 1930- Macmillan, 

 London. 



age, Aquaeductus cochleae ; bm, bony meatus ; c, cochlea ; ca, cartilage ; d, chorda tympani ; em, external 

 meatus ; enl, saccus endolymphaticus ;/r, fenestra rotunda ; im, foramen for auditory nerve ; p, pinna ; 

 pi, perilymph ; po, periotic ; s, sacculus ; sc, semicircular canal ; si, suspensory ligament ; sq, squamosal ; 

 U, attachment of tympanic muscle ; ty, tympanum ; u, utriculus ; i, stapes ; 2, incus ; 3, malleus. 



parts of the inner ear, which themselves contain endolymph. 

 Muscles attached to the ossicles reduce their movement on 

 receipt of a loud sound, so that there is some automatic volume 

 control. The Eustachian canal, which leads from the middle ear 

 to the pharynx, prevents excessive differences of pressure on the 

 two sides of the tympanum. Its pharygeal end is normally closed, 

 but opens in the act of swallowing. The part of the ear sensitive 

 to sound is the cochlea, a long spirally-coiled tube which starts 

 from the sacculus. It is divided internally by membranes into 

 three chambers, which are shown diagrammatically in Fig. 368. 

 The upper, the scala vestibuli, is closed by the fenestra ovalis. 



