3°4 



AMPHIBIA 



xii. 6 



In the anurans the entire skeletal and muscular system has become 

 specialized for the peculiar swimming and jumping methods of loco- 

 motion, by means of extensor thrusts of both hind limbs, acting 



Fig. 177. Reflexes associated with the transition from swimming to walking in toads. 

 The shaded outlines show successive positions as the animal emerges on to solid ground. 

 The first effective contact is by the left fore-limb whose retraction and extension 

 elicits a crossed protraction reflex in the right fore-limb (Lj), a diagonal extensor 

 response in the right hind-limb (L 2 ), and a placing response in the left hind-limb (L 3 ). 

 The right fore-limb then touches the ground and produces corresponding responses 

 Rx- 3 . The left fore-limb in response to stretch of its protractor muscles swings forward 

 and this produces retraction of the left hind-limb (L 4 ) and protraction of the right 

 hind-limb (L 5 ). Fixation of the right hind foot then produces a crossed flexor response 

 (RHJ. (From Gray, J. exp. Biol. 23.) 



together. Frogs, and especially toads, also walk on land, bringing into 

 play a set of myotactic (proprioceptor) reflexes that depend on the 

 contraction of the muscles against an external resistance (Fig. 177). 



The actions of jumping and walking are possible because of pro- 

 found changes in the arrangement of the skeleton and muscles. The 

 myotomal muscles no longer perform their primitive function of pro- 

 ducing metachronal waves of contraction, and accordingly the verte- 

 bral column (Fig. 178) has lost its original flexibility. Instead, it is 



