30 THE BIOLOGY OF THE FROG chap. 



limbs up into a position for making the ordinary swimming 

 stroke. Along with the withdrawal of the body from the 

 surface the fore legs make a sudden stroke backward and 

 upward, thus throwing the anterior end of the body down. 

 Then the hind legs extend and shoot the animal farther 

 downward through the water. The attitude of the body, as 

 the frog rests at the surface, is one of preparation for the act 

 of diving, just as its attitude on the ground is one of readi- 

 ness for a spring. At the moment the frog leaves the sur- 

 face, bubbles of air may generally be seen to escape from the 

 nostrils. 



Righting Movements. — Like most animals, the frog when 

 placed upon its back will regain its normal position. It 

 does so, too, with remarkable quickness, certainly in less 

 than half a second. The operation involves the coordinated 

 action of several muscles. The position of equilibrium may 

 be attained by rolling over either to the right or to the left, 

 and a frog will do now the one and now the other, some- 

 times hesitating a moment between the two courses. A frog 

 will right itself a great many times in quick succession, and 

 in course of time will become so fatigued that it will act 

 slowly enough to give the observer a chance of following its 

 movements. These movements vary a good deal in different 

 acts, but they commonly occur in about the following way : 

 If the frog rolls over toward its left side, the right hind leg is 

 brought dorsally by a contraction of the muscles of the dorsal 

 side of the thigh ; the muscles of the ventral side of the left 

 thigh also contract ; both these movements tend to roll the 

 body over to the left. The right hind leg is often brought 

 forward so that the thigh lies at a considerable angle from 

 the body, and this gives the limb a greater purchase in roll- 

 ing the body over. The left fore leg is brought down along- 

 side of the body, and the opposite member is thrown over to 



