upon his face, and by alternately flexing and extending his arms and legs, can float and propel himself for long periods with perfect safety and with comparatively little exertion. He can also assume the vertical position, and by remaining perfectly motionless, or by treading the water with his feet, can prevent himself from sinking; nay more, he can turn a somersault in the water either in a forward or backward direction. The position most commonly assumed in swimming is the prone one, where the ventral surface of the body is directed towards the water. In this case the anterior and posterior extremities are simultaneously flexed and drawn towards the body slowly, after which they are simultaneously and rapidly extended. The swimming of the frog conveys an idea of the movement.[51] In ordinary swimming, when the anterior and posterior extremities are simultaneously flexed, and afterwards simultaneously extended, the hands and feet describe four ellipses; an arrangement which, as explained, increases the area of support furnished by the moving parts. The ellipses are shown at fig. 38; the continuous lines representing extension, the dotted lines flexion. [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. [Illustration: Fig. 38.] [Illustration: Fig. 39.] [Illustration: Fig. 40.] 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_. [Illustration: 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._