STRUCTURE OF FISHES. 



\9l 



(bbbbb) the other, parallel with the spines, to which the 

 rays are articulated (cc). The fin has also a lateral mo- 

 tion, by which it is occasionally drawn out of a straight 

 line; and by the co-operation of these muscles on both 

 sides, it is kept steady whilst the body of the fish is 

 turned oblique in swift motion, or in eddies. When 

 placed near the tail, the single fins seem also to aid the 

 effect of that instrument by increasing its breadth. 



The tail is the principal organ of progressive motion, Explanation of 

 and its actions are performed by the great mass of lateral ^jj^ action of 

 muscles. There are a series of short muscles for the pur- 

 pose of changing the figure of the tail fin, which arise 

 from the spine and coccyx^ and are attached to the rays 

 immediately beyond their joints : {dd): their action is to 

 expand the rays, and by partial contractions to alter the 

 lateral position of the fin. Slender muscles are placed 

 between the several rays, (ee,) whose office is to converge 

 them previous to the stroke of the tail. 



The muscles situated on the head are those, which act 

 on the membrana branchiostega^ the under jaw, os hyoi" 

 des^ fauces^ and the globe of the eye. 



In order to determine the effect of the fins on the mo- Experiments 

 tions of fishes, a number of living dace*, of an equal size, o" i^Jf j^^"*^^ 

 were put into a large vessel of water. The pectoral fins ^^ ofcthcir^fiai^ 

 of one of these fishes were cut off, and it was replaced 

 with the others. Its progressive motion was not at all 

 impeded ; but the head inclined downward, and when it 

 attempted to ascend, the effort was accomplished with 

 difficulty. 



The pectoral and abdominal fins were then removed 

 from a second.fish. It remained at the bottom of the 

 vessel, and could not be made to ascend. Its progressive 

 motion was not perceptibly more slow ; but when the tail 

 acted, the body shewed a tendency to roll, and the singl* 

 fins were widely expanded, as if to counteract this effect. 



From a third fish, the single fins were taken off. This 

 produced an evident tendency to turn round, and the pec- 

 toral fins were kept constantly extended to obviate that 

 motion. 



• Cyprinvt* Ic\i«Ucu8. 

 Vol. XV.—NoT. 1806. Dd From 



