XII. 27 (353) 



27. The ear of Amphibia 



The inner ear is divided into a utricle, from which the semicircular 

 canals arise, and a saccule, from which there is a diverticulum, the 

 lagena, part of whose receptor surface is covered with a tectorial 

 membrane somewhat similar to that of mammals. There is, however, 

 no coiled cochlea. The middle ear of the frog consists of a funnel- 

 shaped tympanic cavity communicating with the pharynx and closed 

 externally by a tympanum supported by a tympanic ring. Sound 

 waves are transmitted across the cavity by a rod, the columella, fitting 

 by an expanded foot, the otostapes, into the fenestra ovalis, a hole in 

 the wall of the auditory capsule. This hole is also partly occupied by 

 a second plate, the operculum, which is joined to the scapula by a 

 special opercular muscle. The operculum and otostapes develop with- 

 in the wall of the auditory capsule and the middle part of the colu- 

 mella (mediostapes) forms as an outgrowth from the otostapes. The 

 outer part of the columella (extra-columella) and the tympanic ring 

 develop close to the quadrate and probably from its cartilage. 



The columella, therefore, shows no developmental relationships to 

 the hyoid arch. The tympanic cavity is developed from the spiracular 

 cleft, after a strange series of changes. The original cleft degenerates 

 six days after hatching but about six of its lining cells persist and at 

 the end of the tadpole stage form a tympanic vesicle, which becomes 

 connected with the pharynx by a rod of cells. This rod then degener- 

 ates again and an open air passage to the vesicle of the drum is not 

 established until some thirty days after emergence from the water, 

 when a pouch from the pharynx joins the tympanic cavity. These 

 events show the complexities that may result from the modification 

 of developmental processes, and they emphasize the difficulty in 

 assigning 'homologies'. It is still debated to what extent the middle 

 ear of the frog can be compared with that of amniotes. The hyo- 

 mandibular nerve, which divides above the middle ear of amniotes 

 (and above the spiracle of the dogfish) lies behind the tympanic cavity 

 of the frog and branches below it. 



The arrangement for conveying vibrations to the ear varies con- 

 siderably among amphibians. In urodeles there is no tympanum. In 

 some of them the columella is attached to the squamosal, perhaps in 

 connexion with a semi-aquatic or burrowing habit. A similar arrange- 

 ment may have been present in the earliest amphibians, which have 

 a columella but no oval window. In other urodeles (Plethodontidae) 

 the columella is attached to the quadrate and there may be a second 



