Snakes and Frogs and Their Relatives [ 199 



frequently than adults, and healthy individuals more often than 

 those in poor condition. 



How Snakes Move 



If you discover a snake on hard, packed soil, you will notice 

 that it moves very slowly. But once it gets off this smooth surface 

 into long grass or on rough ground, it will whisk out of sight with 

 surprising swiftness. 



Observe a captive snake and you will see the reason for this 

 change in pace. When it is placed on a large piece of glass, it 

 will slip and slide ineffectively; but if it is transferred to loose 

 sand, it is immediately able to make progress. This it does with 

 sideways movements, keeping the full length of its body against 

 the ground. 



You will observe that the creature leaves in its wake a series of 

 slightly curved piles of sand; these prove to be pivots the reptile 

 has raised in pushing its body forward. Next watch a snake on 

 rough grass. Here it will travel with even greater ease each blade 

 of grass serves as a pivot. 



"Legs" Without Feet: To produce this undulating movement 

 apparently its most usual method of traveling the snake depends 

 largely on its muscles and ribs which, in effect, are footless legs. 

 The ribs are attached to the backbone and also to the muscles 

 and the slightly overlapping scales on the underpart of the body. 

 When the muscles are moved forward, the scales are carried 

 forward also. When the muscles are pulled back, the lower edges 

 of the scales press and catch against any roughness on the surface 

 over which the snake rests, and push it forward, using the rough 

 spot for leverage. The snake does not move muscles all along its 

 body at the same time; instead, it brings them forward gradually, 

 and the scales move in waves. 



Side-winding: Another method snakes occasionally employ is to 

 curve the body into an S form and then straighten it out again, 

 pushing forward a little in the process. Some desert snakes have 

 developed a specialized method called "side-winding" which is 

 practically indescribable except that the body is thrown into great 



