[51] The frog in swimming leisurely frequently causes its extremities to move diagonally and alternately. When, however, pursued and alarmed, it folds its fore legs, and causes its hind ones to move simultaneously and with great vigour by a series of sudden jerks, similar to those made by man when swimming on his back.
Thus when the arms and legs are pushed away from the body, the arms describe the inner sides of the ellipses (fig. 38, _a a_), the legs describing the outer sides (_c c_). When the arms and legs are drawn towards the body, the arms describe the outer sides of the ellipses (_b b_), the legs describing the inner sides (_d d_). As the body advances, the ellipses are opened out and loops formed, as at _e e_, _f f_ of fig. 39. If the speed attained is sufficiently high, the loops are converted into waved lines, as in walking and flying.--(_Vide_ _g g_, _h h_ of fig. 40, p. 81, and compare with fig. 18, p. 37, and figs. 71 and 73, p. 144.) The swimming of man, like the walking, swimming, and flying of animals, is effected by alternately flexing and extending the limbs, as shown more particularly at fig. 41, _A_, _B_, _C_.
FIG. 41.--_A_ shows the arms and legs folded or flexed and drawn towards the mesial line of the body.--_Original._
_B_ shows the arms and legs opened out or extended and carried away from the mesial line of the body.--_Original._
_C_ shows the arms and legs in an intermediate position, _i.e._ when they are neither flexed nor extended. The arms and legs require to be in the position shown at _A_ before they can assume that represented at _B_, and they require to be in the position shown at _B_ before they can assume that represented at _C_. When the arms and legs are successively assuming the positions indicated at _A_, _B_, and _C_, they move in ellipses, as explained.--_Original._]
By alternately flexing and extending the limbs, the angles made by their several parts with each other are decreased and increased,--an arrangement which diminishes and augments the degree of resistance experienced by the swimming surfaces, which by this means are made to elude and seize the water by turns. This result is further secured by the limbs being made to move more slowly in flexion than in extension, and by the limbs being made to rotate in the direction of their length in such a manner as to diminish the resistance experienced during the former movement, and increase it during the latter. When the arms are extended, the palms of the hands and the inner surfaces of the arms are directed downwards, and assist in buoying up the anterior portion of the body. The hands are screwed slightly round towards the end of extension, the palms acting in an outward and backward direction (fig. 41, _B_). In this movement the posterior surfaces of the arms take part; the palms and posterior portions of the arms contributing to the propulsion of the body. When the arms are flexed, the flat of the hands is directed downwards (fig. 41, _C_). Towards the end of flexion the hands are slightly depressed, which has the effect of forcing the body upwards, and hence the bobbing or vertical wave-movement observed in the majority of swimmers.[52]