20C 



ANATOMY OF VERTEBRATES. 



Other dismemberments of the cranial myocommas are specialised 

 to act upon the branchiostegal appendages, the branchise, the 

 ujiper and lower jaws, &c. ; and the chief of these, under their 

 special denominations will next be noticed. 



The upper and lower jaws are so connected together in Osseous 

 Fishes that one cannot be moved without affecting the other, and 

 both are alike moveable. Protrusion and retraction affect them 

 equally, and usually to a greater extent than divarication and ap- 

 proximation, or the opening and sliutting of the mouth : in a 

 minor degree, also, the two halves of both maxillary and mandibu- 

 lar arches have transverse movements, varying the angle at which 

 they severally meet at the premaxillary or premandibular symphysis. 

 The most important retractor, wliich tends in that action also to close 

 the mouth, is the large subquadrate muscle, retractor oris, fig. 134, 

 20, 20, which arises from the tympanic pedicle and anterior border 

 of the preoperculum, and is inserted by the upjDcr fasciculus into 

 the maxillary ; by a lower fasciculus into the mandible behind the 

 coronoid process ; and by an aponeurosis into the membrane uniting 

 the two jaws near the angle of the mouth. The muscle which 

 tends to open the mouth by depressing the mandible, on which it 



135 



Lriwoi- niii.'jrles of hmd jiiKl fins. Perch, xxxrii. 



exclusively acts, is that marked 27 in fig. 135 ; it arises from the 

 ceratohyal, and is inserted into tlie back part of the dentary, near 

 the sympliysis. Cuvier deems it the homologue of the qrniohy- 

 oideus. Above tlie insertions of tlic geniohyoid pair is a muscle, 

 the interynandib'idaris, fig. 135, 21, wliich passes transversely from 

 one dentary to the other, approximating the halves of the man- 



