350 Henry L. Bruner 



external naris. Marked differences appear between the two muscles, 

 it is true, as they advance toward the external nasal opening; but 

 such differences are easily explainable as the result of a change of 

 function of the dilatator of Rana. This change brought with it a 

 diminished demand for muscular activity, led to a reduction^ of the 

 dorsal (rostral) portion of the muscle to connective tissue, and explains 

 at the same time the insertion of this connective tissue on the median 

 margin of the external naris. 



In view of the above facts, then, it is entirely certain that the 

 muscle, hitherto assumed to be a dilatator naris in Rana, is in fact 

 a homologue of the dilatator naris of Triton. The degeneration and 

 change of function of the muscle in Rana has followed as a natural 

 consequence of the introduction of a new mechanism for opening the 

 external naris. 



The homology of the musculus constrictor naris of Rana is not 

 so easy to determine with certainty. In its relation to the dilatator 

 naris, it resembles the constrictor naris of Triton on account of its 

 more superficial position; moreover the medial end of the two muscles 

 possesses in part the same insertion. In view of these facts, and 

 because a constrictor naris most probably existed in the ancestors 

 of Rana, it seems reasonable to assume that the muscle under 

 consideration is a constrictor naris, which has reached its 

 present condition by degeneration of its lateral portion, 

 which became superfluous with the introduction of a new 

 apparatus for closing the external naris. I shall therefore 

 consider this muscle to be homologous with the constrictor 

 naris of Triton. 



b. The muscles of the older authors. 

 The first attempt to describe special muscles for the opening 

 and closing of the external naris of the frog was made by Zenker 

 (56), whose work appeared in 1818. Zenker enumerates the fol- 

 lowing muscles: 1) a single m. intermaxillaris, which lies between 

 the facial processes of the two intermaxillaria and opens the external 

 nares; 2) two paired mm. intermaxillares, one on each side lateral 

 from the facial process of an intermaxillare. This muscle closes 



1 According to the researches of Flemming (18) such a reduction is easily 

 explained. 



