166 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



these great travelers are aided by their bilateral symmetry, their light internal 

 skeletons, and their muscles. 



In fishes the muscles of the body wall are usually divided into segments 

 (myomeres). They carry on the main work of locomotion; fins do not do the 

 heavy work. Fishes move by a sidewise undulation, a wavy motion with mus- 

 cles contracting first along one side of the body, then along the other (Fig. 

 10.9). They push their bodies with their tails giving the main drive. Spotted 



Fig. 10.9. A fish swims by undulating movements and 

 pushes its body forward by pressing against the water, 

 successive waves of curvature traveling backward along 

 the trunk and tail. (Redrawn from Romer: The Verte- 

 brate Body. Philadelphia, W. B. Saunders Co., 1949.) 



newts (Triturus) spend one period of their lives in water and another on land, 

 but they move with the wavy swing of the fishes all their lives (Fig. 34.2). 



Land vertebrates have a different problem; they cannot push against the air 

 with their bodies for air is too yielding; they must push the ground or its 

 counterpart. Their movement on four legs from one place to another is the 

 great achievement of their voluntary muscles and partner bones and nerves 

 (Fig. 10.10). Man's movement on two legs is still more difficult and more 

 significant in that it has left the muscles of his hands and arms free to use 

 tools. Standing on two legs is a continued balance which requires that a large 

 number of muscles be kept in sustained contraction in reply to impulses that 

 recur because of the stimulation of the sensory receptors of position (proprio- 

 ceptors). In contrast to the healthy resilience of upright posture is the com- 

 plete limpness of muscles that follows poliomyelitis. The muscles are still nor- 

 mal but the motor nerve cell bodies in the spinal cord have been attacked by 

 the virus. Walking on two legs involves holding the body and head upright and 

 shifting the entire weight to the hind limbs, thus freeing the front ones. With 

 the start of walking the body falls forward, then one leg, say the left one, is 



