A STUDY OF THE MUSCLES 207 



Fishes usually have two pairs of fins that correspond in a 

 way to the arms and legs of man. Their principal use 

 is to steer the animal through the water, for most of the 

 force for the forward movement is supplied by the muscles 

 that move the tail from side to side. Such rapid movement 

 is possible by this means of locomotion that the salmon is 

 able to travel twenty miles an hour. 



Frogs in swimming use their hind legs almost wholly. 

 All the joints are first flexed, thus doubling the appendages 

 near the body ; they are then quickly straightened by the 

 strong extensor muscles, and the push of the webbed feet 

 against the water drives the body forward in a succession 

 of jerks. Swimming and wading birds (ducks, flamingoes) 

 are likewise provided with webbed feet. Alligators and 

 crocodiles swim with their tails, like fishes. 



Locomotion in the Air. Animals that fly require a much 

 more highly developed muscular system than do those that 

 swim. The water buoys up a fish and furnishes a dense 

 medium for the fins and tail to push against. Air, on the 

 other hand, is eight hundred times lighter than water. We 

 have seen that the skeleton of a bird is made light by the 

 air cavities within the bones. The skeleton of a crow 

 weighs when dried only one three-hundredth of a pound. 



The wings of a bird are anterior appendages wonderfully 

 adapted for locomotion in the air, since great extent of sur- 

 face is secured by the expanse of feathers, without adding 

 materially to the weight of the body. The powerful 

 muscles that cause the quick downward movement of the 

 wings are attached to the breastbone. This has the form 

 of a ship's keel in flying birds and serves to cut the air, 

 while the tail acts like a rudder to steer the bird. The 

 hawk is able to fly at the rate of one hundred and fifty 

 miles an hour. 



In bats the long, slender finger bones are covered over 

 and connected with one another by a thin skin, and thus 

 there is formed a very broad but light kind of wing. The 



