Chap, xi LI EAR OF VERTEBRATA. 467 



remarkable arrangement, by means of which sounds are 

 conveyed to the organ of hearing. The anterior ends 

 of the air bladder are attached to membranes which 

 close in spaces in the occipital region of the skull, 

 and on the other side of these membranes is the ear of 

 that side. This simple condition which obtains in the 

 perch and its allies, is complicated in carps and others 

 by the addition of three ossicles, which connect the air 

 bladder with the auditory region, and convey such 

 vibrations as affect the air in the air bladder. 



In the lower Amphibia the ear cleft is merely 

 closed by muscles, but in the Anura there is a distinct 

 tympanic membrane, as there is also in most, though 

 not in all, Reptiles, In the simplest condition this 

 lies on the surface of the head, so that there is no ex- 

 ternal auditory meatus ; but in the lizards we have a 

 small pit external to the membrane, and we have, 

 therefore, the commencement of an external ear 

 passage. 



In Mammals the tympanic bone of the skull 

 takes part in forming the walls of this meatus. The 

 pinna is represented by a fold of skin with combined 

 muscular tissue in crocodiles, and by a movable mem- 

 branous valve in some birds (owls). It is not found in 

 the Prototheria, but in all other Mammals there is a 

 well-developed outer ear, which becomes rudimentary 

 or aborted only in marine forms. In many mammals 

 the pinna can be moved by muscles, and directed, 

 therefore, to different points from which sound is sup- 

 posed to be coming. In the higher Primates these 

 muscles are ordinarily rudimentary, but their posses- 

 sion by some men is spoken to by the power that such 

 have of moving their ears. 



Middle car. Associated with the development 

 of a tympanic membrane is that of a contained tym- 

 panic cavity. This cavity is not a new formation, but 

 is due to the modification of the, in the pulmonate 



