THE RELATION OF THE CHORDA TYMPANI TO 

 THE VISCERAL ARCHES IN MICROTUS. 



By Victor E. Emmel. 



(Conlri/mted from the' Biologiral Laboratory of Pacific University, under the 

 direction of G. E. Coghill). 



The mammalian chorda tympani is a branch of the facial 

 nerve which passes over the tympanic cavity, underneath the 

 auditory ossicles and joins the lingual branch of the trigeminus. 

 It is generally accepted that the tympanic cavity and auditory 

 ossicles are derivatives of the spiracular cleft and visceral arches 

 of fishes. It would seem a natural conclusion, therefore, that 

 the chorda tympani is also homologous with the pre spiracular 

 branch of the facial nerve of fishes and amphibians. Upon 

 this point, however, authorities are not agreed. On the one 

 hand, a large number of investigators regard the chorda tym- 

 pani as the homologue of the pre-spiracular (pre-trematic) nerve 

 of fishes, as, for example, Balfour, in describing the anterior 

 branch of the seventh nerve of Elasmobranchii, says : ' 'This 

 branch forms the prae-spiracular nerve of the adult and is 

 homologous with the chorda tympani of mammals" (Compara- 

 tive Embryology, Vol. II, p. 459). Strong, in his work on 

 the cranial nerves of Amphibia, interprets the r. mandibularis 

 internus of fishes and Anura, and the r. alveolaris of Urodela, 

 as homologous with the mammalian chorda tympani. On the 

 other hand, Druner denies that the r. alveolaris is the homo- 

 logue of the chorda tympani {Zool. JaJirb., XV, 3); while Cog- 

 hill, in his work on the cranial nerves of Amblystoma, inter- 

 prets the r. alveolaris of Urodela as pre-spiracular, and takes 

 the tentative position that the "most complete morphological 

 and physiological representative (of the chorda tympani) in the 



