COMPARATIVE ANATOMY. 
136 
their action mechanical, their position, and the muscles by 
which they are moved, in the highest degree convenient. 
The following short account of some experiments upon fish, 
made for the purpose of ascertaining the use of their fins, will 
be the best confirmation of what we assert. In most fish, 
beside the great fin, the tail, we find two pair of fins upon 
the sides, two single fins upon the back, and one upon the 
belly, or rather between the belly and the tail. The baU 
ancing use of these organs is proved in this manner. Of 
the large-headed fish, if you cut off the pectoral fins, i. e. 
the pair which lies close behind the gills, the head falls 
prone to the bottom; if the right pectoral fin only be cut 
off, the fish leans to that side; if the ventral fin on the 
same side be cut away, then it loses its equilibrium entire¬ 
ly; if the dorsal and ventral fins be cut off, the fish reels 
to the right and left. When the fish dies, that is, when 
the fins cease to play, the belly turns upwards. The use 
of the same parts for motion is seen in the following obser¬ 
vation upon them when put in action. The pectoral, and 
particularly the ventral fins, serve to raise and depress 
the fish: when the fish desires to have a retrograde motion, 
a stroke forward with the pectoral fin effectually produces 
it; if the fish desire to turn either way, a single blow with 
the tail the opposite way, sends it round at once: if the 
tail strike both ways, the motion produced by the double 
lash is progressive , and enables the fish to dart forwards 
with an astonishing velocity.* The result is not only in 
some cases the most rapid, but in all cases the most gen¬ 
tle, pliant, easy animal motion with which we are acquaint¬ 
ed. However, when the tail is cut off, the fish loses all 
motion, and gives itself up to where the water impels it. 
The rest of the fins, therefore, so far as respects motion, 
seem to be merely subsidiary to this. In their mechanical 
use, the anal fin may be reckoned the keel; the ventral 
fins, out-riggers; the pectoral muscles, the oars: and if 
there be any similitude between these parts of a boat and 
a fish, observe, that it is not the resemblance of imitation, 
but the likeness which arises from applying similar me¬ 
chanical means to the same purpose. 
We have seen that the tail in the fish is the great in¬ 
strument of motion. Now, in cetaceous or warm-blooded 
♦Goldsmith’s History of Animated Nature, vol. iv. p. 154. The 
velocity with which fish swim from one part of the globe to another is 
astonishing; when a ship is sailing at the rate of fourteen miles an hour, 
the porpoises will pass it with as much ease as when at anchor. 
Paxton . 
