Archive image from page 451 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana03todd Year: 1847 438 MOTION. the intensity of their action (all oilier things re- maining the same) must be proportional to their areas respectively. Sometimes the Rays glide sideways, in which motion the pectoral and caudal fins exchange their office, the former striking horizontally, and the latter vertically, the result of which may be obtained by the composition of forces, when their directions and intensities are given. The Rays, being destitute of an air-bladder, require
Archive image from page 451 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana03todd Year: 1847 438 MOTION. the intensity of their action (all oilier things re- maining the same) must be proportional to their areas respectively. Sometimes the Rays glide sideways, in which motion the pectoral and caudal fins exchange their office, the former striking horizontally, and the latter vertically, the result of which may be obtained by the composition of forces, when their directions and intensities are given. The Rays, being destitute of an air-bladder, require a much greater force in the vertical direction upwards to sustain themselves in swimming; hence the necessity for the power and mobility of the pectoral fins which we find conferred on them. The great lateral development of the surface of the Rays compared with their depth, and the great depth of the Pleuronectides compared with their breadth, entitle the former, rather than the latter, as Mr. Yarrell justly observes, to the appellation of flat fish. The first movement of a fish from a state of rest is usually produced by the flexion of its tail, as to a, Jig. 232 ; during this action the centre of gravity (c) recedes slightly from its previous position ; the tail being flexed into the position «, is forcibly extended by the muscles on the opposite side, in the direction of the line a i, perpendicular to its plane. The force of its action upon the water in a i is translated to the body of the fish in i «, causing the centre of gravity c to move obliquely forwards in the direction of c h, parallel to i a. The tail having reached the mesial line c dt its power of urging the centre of gravity forwards not only ceases, but during its flexion in e 0, it acts backwards in the direction of o e; having reached the point o, it is again forcibly ex- tended in the line o f, causing an impulse on the centre of gravity in c b, parallel to o e; if the two forces c li and c b acted simul
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