3 
and mechanical Action of the Muscles of Fishes. 
and for keeping the back upwards. The single fins appear 
to prevent the rolling of the body, whilst the tail is employed 
to impel it forward. 
Each of those fins, which are in pairs, is capable of four 
motions, viz. of flexion and extension, like oars, and of ex- 
panding 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 great power of 
the extensor muscle ( Vide Plate I. a, a,) shews how strongly 
it is required to act when employed to stop suddenly the pro- 
gressive 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 expanded, and the effect is assisted by 
the tail turning laterally 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; (bbbbh) the other, 
parallel with the spines, to which the rays are articulated ( cc.) 
The fin has also a lateral motion, 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. 
B 2 
