64 LOCOMOTION. 



move in separate lines, the body has also a lateral^ 

 vibratory motion. A man, in walking, puts down one 

 foot before the other is raised, but not in running. 

 Q-uadrupeds, in walking, have three feet upon the 

 ground for most of the time ; in trotting, only two. 

 Animals which walk against gravity, as the conimon 

 fly, the tree-toad, &c. support themselves by suction, 

 using cavities on the under side of their feet, which 

 they enlarge at pleasure, till the pressure of the atmos- 

 phere causes them to adhere. In other respects, their 

 locomotion is effected like that of other walking 

 animals. Birds perform the motion of flying by striking 

 the air with the broad surface of their wings in a 

 downward and backward direction, thus propelling the 

 body upward and forward. After each stroke, the 

 wings are contracted, or slightly turned, to lessen their 

 resistance to the atmosphere, then raised and spread 

 anew. The downward stroke also, being more sudden 

 than the upward, is more resisted by the atmosphere. 

 The tail of birds serves as a rudder to direct the course 

 upward or downward. When a bird sails in the air 

 without moving the wings, it is done, in some cases, by 

 the velocity previously acquired, and an oblique direc- 

 tion of the wings upward ; in others, by a gradual 

 descent, with the wings slightly turned in an oblique 

 direction downward. Fishes, in swimming forward, 

 are propelled chiefly by strokes of the tail, the extremity 

 of which being bent in an oblique position, propels the 

 body forward and laterally at the same time. The 

 lateral motion is corrected by the next stroke, in the 

 opposite direction, while the forward course continues. 

 The fins serve partly to assist in swimming, but chiefly 

 to balance the body, or keep it upright ; for the centre 

 of gravity being nearest the back, a fish turns over 

 when it is dead or disabled. Some other aquatic 

 animals, as leeches, swim with a sinuous or undulating 

 motion of the body, in which several parts at once are 

 made to act obliquely against the water. Serpents, in 



