PROGRESSION ON AND IN THE WATER. 



81 



direction. The position most commonly assumed in swim- 

 ming 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.^ In ordinary swimming, when the 

 anterior and posterior extremities are simultaneously flexed, 

 and afterwards simultaneously extended, the hands and feet 

 describe four ellipses j 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 repre- 

 senting extension, the dotted lines flexion. 



Fig. 3& Fig. 39. 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, ff of fig. 39. If the speed 

 attained is sufficiently high, the loops are converted into 



1 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. 



5 . 



