MOTION. 
lighter than water, and as the specific gravities of 
airand water are to each other nearly as 1 to 815, 
asmall bulk is sufficient to render the lesser 
fishes lighter than the medium they inhabit. 
The position of the air-bladder being immedi- 
ately under the spine and above the centre of 
gravity causes the fish to rise without the danger 
of turning over on its back. Those fishes which 
are furnished with an air-bladder are capable 
_ Of either renewing, expelling, compressing, or 
dilating its aerial contents, and of varying its 
area so as to rise, sink, or remain in equili- 
brio. The air-bladder becomes by this means 
an important auxiliary organ of locomotion, 
_ and affords an illustration of one of the many 
__ evidences of design in the primary formation of 
_ aquatic animals. 
_ The Diodons and Tetrodons render them- 
selves buoyant by swallowing air, which filling 
the first stomach becomes inflated like a bal- 
loon; but as the gastric reservoir lies below 
‘the centre of gravity, the bodies roll over in 
_ an inverted position, and are driven in the 
direction of the winds and tides without the 
abl of directing their course.* The forms of 
fishes are considerably diversified, being sphe- 
_ fical in the globe ¢tetrodon ; an elongated cy- 
linder in the Eel; compressed in the Dory and 
_ Spah ; flattened into planes parallel to the me- 
‘sial section in the Pleuronectide ; elliptical in 
the Salmonide, Scomberide, and Mugilide. 
‘In nearly all the orders of fishes the surface 
laa to the water by the head and shoul- 
ders inclines more or less to the vertebral axis 
_ of the fish, which coincides with the axis of 
_ Motion, and therefore is adapted to offer resist- 
‘ance, which varies with the angle of incli- 
‘nation. 
In the Salmon, Cod, and Mackerel the form 
of the body approximates to that which is con- 
_ sidered by mathematicians to offer the least re- 
_ Sistance to the surrounding medium. The 
_ Organs of support are developed principally in 
_ the plane of the mesial section, and consist 
_ of superior and inferior spinous, interspinous, 
dorsal, and ventral fin elements, the projections 
Of which prevent motion of the vertebral axis 
im the plane of the mesial section. The ver- 
tebre are short, numerous, and, towards the 
adal extremity, destitute of transverse pro- 
es, an arrangement which gives the tail 
‘considerable degree of lateral motion; 
ving to which it becomes the most essential 
can of locomotion. The locomotive or- 
ms of fishes are the fins and tail; the 
toral fins represent the anterior, and the 
entral the posterior extremities of the higher 
ers of Mammalia. In the Cod, the legs 
e absolutely in front of the arms, being sus- 
ded under the throat. The Percide, which 
_ are provided with two dorsal, two pectoral, and 
two ventral, as well as anal and caudal fins, 
“have the greatest number of locomotive organs. 
The planes of the dorsal and anal fins are in 
the mesial section of the fish, and being res- 
tricted in that plane by a kind of ginglymoid 
joint, are capable only of elevation and depres- 
_* See Dr. Roget’s Bridgewater Treatise. 
437 
sion. In the Cod, Halibut, and Gurnard, the 
action of these fins serves merely to increase 
or diminish the lateral surfaces of the fish, so 
as to prevent any tendency in the animal either 
to oscillate laterally, or turn upon its vertebral 
axis into an inverted position, which it would 
be inclined to do without some muscular 
effort, since in the erect posture the centre 
of gravity lies above the centre of figure.* 
The plane of each ventral fin is in general 
nearly horizontal, and perpendicular to that of 
the caudal ; their action serves to balance the 
body, to incline it on either side, when one fin 
only acts, and to elevate and depress the fish 
by their joint effort+ In many fishes the 
pectoral fins being at right angles to the tail 
and vertical, act horizontally, and communicate 
either a progressive or a retrograde impulse to 
the body, thus assisting the action of the tail ; 
if they are both retained in an extended position, 
they will retard the velocity of the fish; if one 
pectoral fin only is extended, it will turn the fish 
ma curve towards that side ; if the other only, it 
will turn it on the opposite side: they thus per- 
form the office of a rudder. When the planes 
of the pectoral fins are directed obliquely for- 
wards and upwards, they communicate an as- 
cending and a retarding impulse to the fish, but 
the amount of retardation is compensated by- 
the power which the fish acquires of ascending. 
When the caudal, ventral, and oblique pec- 
toral fins move simultaneously, there result 
three forces acting in different planes, whose 
intensities, estimated in directions perpendi- 
cular to those planes, are severally proportional 
to the products of their areas multiplied into 
the squares of their velocities ;{ the resultant of 
these forces may be obtained by the law of the 
parallelogram of forces.§ 
In the Rays, the pectoral fins are developed 
to an enormous extent, and being directed 
horizontally, their action is vertical, like the 
wings of a bird. They are furnished with a 
great number of joints, which endow them with 
considerable mobility ; they have the power to 
increase the surface of the fin during depression, 
and to diminish it during elevation. The disc 
of the ventral fins lies in the same plane as the 
pectoral, and acts in a similar manner, but the 
plane of the caudal fin is at right angles to 
them. The depression of the pectoral and 
ventral fins elevates the fish, whilst the lateral 
motions of the tail propel it forwards. The 
area of the pectoral fins in the Rays is very 
great Compared with that of the caudal; and 
* In Piscibus, pars gravissima ossium spine, 
copiosissima caro musculosa in dorso supremo 
posita est, vesica vero aerea in infimo ventre recon- 
ditur; ergo centrum gravitatis Piscium supra cen- 
trum magnitudinis eorum in supremo dorso repo- 
situm est; et ideo, dum in aqua innatant, naturali 
instinctu revolverentur ventre supino, qua positura 
cum natatui valdé iacommoda sit, cojuntur Pisces 
artificiosé se retinere situ erecto. Borelli, loco cit. 
+ Pinnz duplicate, que in duobus locis infimi 
ventris piscium existunt, non inserviunt ad motum, 
sed ad stationem eorum. Borelli, loco cit. p. 257, 
{ Sce resistance of fluids. 
§ See method of rectangular co-ordinates. 
