196 



STRUCTURE OF FISHES. 



fins, is simple ; and as it is chiefly eflfected by the lateral 

 flexure of the sjjine and tail, upon which the great mass 

 of its muscular flesh is employed, whilst the fins are moved 

 by small muscles, and those, from their position, compa- 

 ratively but of little power, I shall only describe in de- 

 tail the arrangement and application of those masses, 

 which constitute the principal moving organs. 

 — explained For this purpose a well known fish, the cod*, has 



tureofthccod! ^^^" selected as a standard of comparison for themu-^cles 

 of other fishes, there being a conspicuous resemblance 

 among them all. 

 The side fins The pairs of fins have been considered as analogous to 



regulate posi- ^^^*' ^^"^ *^^.^ ^^^ ^"^^ employed for the purposes of 



tion, &c. turning, stopping, altering the position of the fish toward 



the horizon, and for keeping the back upward.-:. The 



single fins appear to prevent the rolling of the body, 



whilst the tail is employed to impel it forward. 



Manner in Each of those fins, which are in pairs, is capable of 



which the fins fouj. motions, viz. of flexion and extension, like oars, 



act, ' ' 



and of expanding the rays, and closing them. 



The extension of the whole fin is performed by a single 

 radiated muscle, which is often supplied with red blood : 

 the antagonist is of a similar character. The greater 

 power of the extensor muscle (Vide Plate V. a, a.) shews 

 how strongly it is required to act when employed to stop 

 suddenly the progressive motion. A series of intervening 

 muscles expand and close the rays. 



In the act of extending the fin the interosseal muscles 

 are passive. It is advanced forward edgeways and closed; 

 but during its flexion, the -rays are expanded, striking 

 the water with its broadest surface : this action assists 

 the tail in turning the fish. In the effort to stop, these 

 fins are strongly retained at right angles with the body, 

 by the force of the extensor muscles, the rays are ex* 

 pandcd, and the efi'ect is assisted by the tail turning later- 

 ally with its broadest surface forward. 



The single fins, for the expansion and contraction of 

 their rays, are furnished with two sets of muscles; one 

 of which is situated at their roots, and lies oblique; 



* Gadus Morhua of Linnaeus. 



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