PISCES FISHES. 497 



nected on the one hand with the articulation of the lower jaw, and 

 on the other with the opercula or gill-covers. These bones are 

 seven in number on each side. 



The Palatine (22) are easily recognizable, forming part of the 

 roof of the mouth, and generally armed with teeth. 



Two bones are connected with the posterior edge of each palate 

 bone : one, situated externally, becomes in reptiles a very impor- 

 tant element, it is called the Transverse bone (24) ; the second 

 (25) is named the Internal Pterygoid. 



The other pieces belonging to this part of the skeleton are not a 

 little interesting on account of their remarkable arrangement ; 

 and, perhaps, the anatomical student will be somewhat startled at 

 the position which some of them occupy. In the first place, the 

 squamous portions of the temporal, instead of entering into the 

 formation of the cranium, are here slightly displaced, and, although 

 still called the Temporal bones (23), are articulated by a hinge- 

 joint with the posterior frontal and mastoid bones, and thus 

 form a moveable basis to which the opercular apparatus is at- 

 tached. 



Connected with the Temporal we have the broad and flat 

 piece (27) which is the Tympanic bone, and to these the pieces 

 forming the opercula are appended. 



Lastly, supporting the lower jaw we find the Jugal bones ; 

 and connecting these with the rest of the temporal apparatus are 

 two small ossicles (31), which complete this portion of the ske- 

 leton. 



The seven bones above enumerated are almost immoveably con- 

 nected with each other by the interposition of cartilage between 

 their edges, a mode of articulation distinguished by the name of 

 synchondrosis ; but the whole apparatus moves readily upon the 

 two hinges, one formed by the articulation of the palate bone with 

 the maxillary and vomer, and the other by the joint which unites 

 the temporal bone to the posterior frontal. This movement, by 

 opening the gill-covers, enlarges the cavity of the mouth when 

 the fish wishes to take in the water necessary for respiration ; or 

 else, by acting in a contrary direction, again expels it. 



(533.) Opercular bones. The great flap, which in osseous 

 fishes closes the gill openings externally, is composed of four 

 pieces, to which the following names have been given. The Pra- 

 operculum (30) is attached to the posterior edge or angle of 

 the palato-temporal apparatus last described, and its borders often 



2 K 



