PISHES. 319 



Another depressor, larger and more extended, is sometimes farther 

 behind, and appears beneath the lateral part of the cranium, before 

 the depressor of the operculum ; in the perch this is wholly a slight 

 sub-division. 



The elevator of this same arch, (No. 24), arises behind the orbit 

 beneath the edge of the posterior frontal, and before the elevator of 

 the operculum ; it is inserted into the upper part of the external sur- 

 face of the temporal, and into a portion of the external pterygoid. It 

 is the antagonist of the former, it draws the palatine arch, and dilates 

 the space destined for the branchiae. 



We find then, that the palatine arch, composed of bones which we 

 have called palatine, pterygoid, internal, external, jugal, box and tem- 

 poral, has motion by its two articulations, the one anterior belonging 

 to the palatine, the other posterior attached to the temporal, and its 

 motion consisting of a drawing away of its inferior part from that of 

 its fellow, or of an approximation of them ; a motion which draws 

 away also the branches of the lower jaw and the opercular apparatus, 

 dilating at the same time the whole of the branchical apparatus. 



This is an action indispensable to the process of respiration, and 

 which is cotemporaneous with the life of the fish. 



Muscles of the Operculum. 



The motions of the operculum are A^ery like those of the palatine 

 arch, and the muscles engaged in these motions are situated behind 

 those of the latter arch. There is also an external one (No. 25), 

 which raises the operculum, and another internal (No. 26), which 

 depresses it, 



They are divided sometimes into several bellies : and in some spe- 

 cies the elevators form two or three distinct muscles. 



The elevator (No. 25) is attached chiefly along the external crest 

 formed by the mastoidean bone ; the depressor (No. 26) is connected 

 with the inferior lateral surface in a part where the great wing and 

 petrous bone are united together and to the mastoidean. It is sepa- 

 rated from the depressor of the palatine arch (No. 24), by the fasci- 

 culus of superior anterior muscles of the branchiae. 



The suboperculum and interoperculum have no proper muscles ; 

 they participate in the general motions made by the palato-temporal 

 arch, and by the operculum properly so called. 



It is proper in this place to remark that the muscles which approx- 

 imate the branches of the hyoid, and contract the branchiostegal 

 membranee also contribute a part in bringing together the palatine 

 and opercular apparatus.* 



Muscles of the Os Hyoides. 



The principal, (No. 27), answers to the genio-hyoidean ; it departs 

 from the internal surface of the branch of the lower jaw near the sym- 

 physis, and supports itself on the sides of the hyoidean branch, into 



* Quite as important a remark is this, that there is no analogy whatever between 

 the muscles of the operculum, and those of the minute bones of the ear in the mam- 

 malia. 



