AUDITORY ORGAN 307 



arches, around the hyomandibular (spiracular) cleft, the lower part 

 of which closes up, while the upper part gives rise to the external 

 auditory meatus. These auricular eminences unite to form a 

 nearly continuous ring, on which are later formed the characteristic 

 protuberances known as the helix, antihelw:, tragus, and antitragus. 

 The variations in form of the pinna which are seen in various 

 Mammals concern essentially the later formed portion (auditory 

 fold), which projects upwards and backwards from the head 

 (Fig. 225). 



The muscles which move the ear as a whole, and which are 

 supplied by the facial nerve, include the following in the majority 

 of Mammals : (a) attrahentes s. adductores, (b) levatores s. attolentes, 

 (c) abductores s. retrahentes, (d) depressores, and (e) rotatores. A 

 gradual reduction of these muscles is seen in the following series : 

 Artiodactyla and Perissodactyla, Canidae, FelidaB, Prosimii, Primates, 

 the reduction being most marked in Man. 1 



1 The auditory fold may undergo marked reduction, e.g. in aquatic and sub- 

 terranean forms. Thus amongst the Pinnipedia, only the Otaridae possess an 

 " external ear." The corresponding muscles become transformed into sphincters 

 for closing the auditory aperture. 



x 2 



