OS HYOIDES AND BRANCHIAL AECHES. 



635 



fourth, placed upon the inner surface of the articular, called the oper- 

 cular, because it corresponds with a bone met with in the lower jaw of 

 reptiles, to which the same name has been applied. The fifth, when 

 present, is very small and unimportant. 



(1723.) Os liyoides and branchiostegous rays. The os hyoides of a 

 fish is situated as in other vertebrate animals ; it is composed of two 

 branches, each made up of several pieces (fig. 313, 37, 38, 39, 40), and is 



Fig. 313. 



Os hyoides and branchial bones of the Perch. (After Cuvier.) 



always suspended from the temporal by means of two small ossicles (59), 

 which, as they represent the styloid process of Man, are called the 

 styloid bones. 



(1724.) Between the two branches of the os hyoides is placed a single 

 central piece (42), which becomes of great importance in Eeptiles and 

 Birds ; and upon this is the bone which supports the tongue, or the 

 lingual bone (41). 



(1725.) The great fissure that exists on each side between the head 

 and shoulder of an osseous fish, wherein the gills are situated, is not 

 closed merely by the opercular bones, but likewise by a broad mem- 

 branous expansion called the branchiostegous membrane, which is ad- 

 herent to the os hyoides, and assists in forming the great valve of the 



