AQUATIC RESPIRATION OF FISHES 21 



takes place mainly in a ventral direction following contraction 

 of the large sternohyoideus muscle which joins the pectoral 

 girdle and basihyal. Lateral expansion of the buccal cavity is 

 effected by contraction of a muscle with its origin on the otic 

 capsule and insertion on the outer surface of the hyomanibula. 

 This muscle is sometimes named the levator hyoidei but it is 

 also frequently termed the levator arcus palatini. This also starts 

 expansion of the opercular cavity; further abduction of the 

 operculum is produced by contraction of the dilator operculi 

 muscles. The opercular cavity also expands ventrally and 

 laterally by the separation of the branchiostegal rays. This 

 movement is produced by one of the hyohyoideus muscles. The 

 relative size of the branchiostegal apparatus varies in different 

 fishes, being highly developed in forms with well-developed 

 suction pumps. 



The decrease in volume of the buccal cavity begins with the 

 relaxation of those muscles concerned in expansion, together 

 with activity of the adductor mandibulae. This muscle is fre- 

 quently divided into four parts, as in the cod, and it is mainly 

 due to the action of parts 2 and 4 that the mandible is closed. 

 Raising of the hyoid arch in the floor of the buccal cavity also 

 plays an important part in the buccal pressure pump. This 

 movement is achieved by contraction of the geniohyoideus 

 (between the basihyal and mandible) when the adductor mandi- 

 bulae is also contracted. Outward expansion of the buccal 

 cavity is antagonised by the action of adductor muscles (adductor 

 arcus palatini* and adductor hyomandibulae). The opercular 

 cavity is decreased in volume by contraction of the adductor 

 operculi and another part of the hyohyoideus muscle. The 

 adductor operculi has its origin in a hollow of the otic region 

 and is inserted on the inner side of the operculum. It functions 

 by a lever system of the third order, which contrasts with the 

 first order lever whereby the opercular cavity increases in volume 

 when the dilator muscle contracts. The latter is inserted on the 

 operculum dorsal and anterior to its articulation with the 

 hyomandibula which forms the fulcrum. 



* Levator arcus palatini of Saunders & Manton 



