FlJSHKS. 311 



existence or absence of the sternum, but which in general is very 

 limited. The bones of the arm are seldom endo\A'ed with any parti- 

 cular mobility. Even the carpus enjoys a separate movement only 

 in the species in which it is prolonged. But all tlie rays have the 

 power of receding from or approaching tOAvards each other, and the 

 fin which is composed of them, has the faculty of moving forward, 

 or of lying flat against the body, or of raising, depressing, and incli- 

 ning differently its plane to the horizon. It acts on the entire fish 

 much in the same way as a wing placed in the same situation, and 

 its strength depends on its surface, and the vigour of its muscles. 



We know that, in the pirabebes and the exocetus, the pectoral fins 

 are large enough to lift the fish out of the Avater, and to enable 

 it to describe in the air an extensive curve. 



The ventral fins follow the motion of the bones of the i^elvis, 

 which move forwards, backwards, and to the sides, and, when not 

 soldered to each other, they recede from or approach each other, 

 l^hey have also a power of expanding or contracting their rays, a 

 combined movement towards the vertical or the horizontal plane, 

 by being drawn towards the belly or the sides; they thus act like oars. 



Lastly the head is slightly moveable on the spine, and largely on 

 the jaws, the palato-temporal arches, the os hyoides, the branchial 

 arches, the pharyngeal bones, and the opercula. The opening or 

 closing of these parts, so useful in deglutition and respiration, con- 

 tributes also to the motion of the fish forwards, by the pressure which 

 the water sustains from them in its passage backwards through the 

 mouth, to be ejected by the openings of the branchiae. 



To these different movements we should add that which the body 

 of the fish receives in the vertical direction, from the greater or lesser 

 degree of compression which the ribs exert on the natatory bladder. 

 This bladder placed under the spine of the back, and filled with air, 

 according as it is more or less compressed or dilated, gives to the 

 fish a specific gravity, equal, superior, or inferior to that of the water, 

 and thus enables it to remain in equilibrium, and either to descend 

 or ascend. 



It noAV remains to explain in this chapter, the muscles which impart 

 the different motions jvist described, to the osseous organs.* 



We shall first describe the great muscles which act on the entire 

 body ; then those of the vertical fins ; we shall then speak of muscles 

 of the fins in pairs ; and lastly, we shall examine those complicated 



* The niyolofy of fishes has heen still infinitely more neglected than their ostoo- 

 lotry. In Gouan's history of fishes, page 69 and followinsr, there is a meagre de- 

 scription of tlie most external muscles taken from an acanthoptcrygian, possihly a 

 spai'us. Vlcq. D'Azyr has given some unfinished sketches of tiie myology of tlie 

 chondropterygians, and the eel (second Memoir en fishes, Acad, des sc. : Sav: ctr. : 

 vol VII.) M. Dumeril and myself have considerably extended t'lcra, in my lectures 

 on comparative anatomy, vol 1. passim for the body and members ; vol. Ill page 90 

 for the jaws, and vol. IV. page 35:; lor the brfincbise and th(ir opercula: but still 

 our descriptions have not all the clearness and generality desirable. Those now 

 given are new, and are taken fiom more numerous observations; as in the rest of 

 this anatomical treatise, I have taken the perch as the pvinoipsd type ; the most re- 

 markable exceptions will be iinticed in their proper pl.'ic;'. 



