MOTION. 



-437 



lighter limn water, and as the specific gravities of 

 air and water are to each other nearly as 1 to 8 1 5, 

 a small 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 

 power of directing their course.* The forms of 

 fishes are considerably diversified, being sphe- 

 rical in the globe tctrodon ; an elongated cy- 

 linder in the Eel; compressed in the Dory and 

 Spah ; flattened into planes parallel to the me- 

 sial section in the Pleuronectida? ; elliptical in 

 the Salmonidse, Scomberida, and Mugilidre. 

 In nearly all the orders of fishes the surface 

 presented 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 

 in the plane of the mesial section. The ver- 

 tebrae are short, numerous, and, towards the 

 caudal extremity, destitute of transverse pro- 

 cesses, an arrangement which gives the tail 

 a considerable degree of lateral motion ; 

 owing to which it becomes the most essential 

 organ of locomotion. The locomotive or- 

 gans of fishes are the fins and tail ; the 

 pectoral fins represent the anterior, and the 

 ventral the posterior extremities of the higher 

 orders of Mammalia. In the Cod, the legs 

 are absolutely in front of the arms, being sus- 

 pended under the throat. The Percidae, 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 Biidgewater Tiea'ise. 



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.f In many rishes 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 

 in a 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 ;J the resultant of 

 these forces may be obtained by the law of the 

 parallelogram of forces. 



Jn the Hays, 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 spina?, 

 copi >sissima caro musculosa in dorso supremo 

 posita est, vesica vcro aerea in infimo venire rrco'i- 

 ditur ; ergo centrum gravitntis Piscium supra cen- 

 trum inagnitudinis eoium in supremo dorso repo- 

 sittim est ; ct ideo, dum in aqua innatant, naturali 

 instinctu revolverentur venire supino, qu* positura 

 cum natatui valde incommoda sit, cojuntur Pisces 

 artificiose se retinere situ erecto. Borelli, loco cit. 

 p. 257. 



f Pinnse duplicate, quae in duobus locis infimi 

 ventris piscium cxislunl, non inserviunt ad mot inn, 

 sed ad stationem eorum. Borelli, loco cit. p. 257. 



\ See resistance of fluids. 



$ See method of rectangular co-ordinates. 



