MYOLOGY OF FISHES. 



•207 



dible after they may have been divaricated. The latter movement 

 depends upon the drawing upward and outward of the tympanic 

 pedicle. This action is performed chiefly by the muscle, levator 

 tympani, figs. 134 and 137, 24, which arises from the postfrontal 

 and expands to be inserted into the epi- and pre-tympanics and 

 into the ectopterygoid. In raising or drawing outward the tym- 

 panic pedicle and attached part of the pterygoid, this muscle tends 

 to dilate the branchial cavity and the back part of the mouth. It 

 is antagonised by the muscle, depressor tympdni, fig. 136, 22, 22, 

 which arises from the basi- and ali-sphenoids, and expands with 

 diverging fibres to be inserted into the epi- and pre-tympanics and 

 into the entopterygoid. It depresses the tympanic, or approximates 

 it to the opposite pedicle, and contracts the branchial cavity. 



The movements of the opercular appendage are like those of its 

 supporting arch, and are performed by muscles placed behind 

 those of that arch. The levator opcrculi, figs. 134 and 136, 25, 

 arises from the mastoid crest, and is inserted into the upper and 

 outer part of the opercular bone. The depressor operculi, fig. 136, 



136 



Muscles of liyoid and operculum, Percli. xxxiu. 



26, arises from the alisphenoid and petrosal, and is inserted into 

 the inner ridge of the opercular bone. The retractor hyoidei, 

 fig. 137, 1 d, fig. 135, c, c, extends from the coracoicl to the uro- 

 and basi-hyals, but is chiefly implanted into the sides of the 



