ANIMAL PHYSIOLOGY 



rents may carry an animal for great distances without any 

 work on its part; while air currents make it possible for 

 birds to soar with little effort. Flight by the vibration of 

 wings, as in birds and insects, requires the greatest expen- 

 diture of energy, since the pushes against the thin air must 

 be made quickly and with great force and be rapidly re- 

 peated to be effective for support and locomotion. Man in 

 making locomotive machines, railway engines, automobiles, 

 steamships, etc., has met the same conditions as the animals; 

 but the difficulties of aerial locomotion are so great that he 

 has only now succeeded in making a beginning toward 

 achieving a mechanism for it. 



The simplest and what may be called the most imperfect 

 modes of locomotion are shown by the simplest animals. 

 These modes we have already studied in Amoeba, Para- 

 mcecium, and Vorticella. The living elements in the body 

 of the higher animals are the many individual cells, and they 



show many kinds of move- 

 ment. But motion in the 

 higher animals is produced 

 chiefly by the contraction of 

 muscles, each of which is 

 made up of contractile fibres 

 which may be thought to be 

 modifications of such a fibre 

 as exists in the stalk of Vor- 

 ticella. 



The muscles require firm 

 points of attachment to pull 

 FIG. 16. Diagram of cross-section against and the complex 



through the thorax of an insect movements of most anima l s 

 to show the exo-skeleton and the 



leg and wing muscles attaching require also rigid levers and 



to it; h, heart; al. c, alimentary fulcra. These firm solid 

 canal; v. n. c. ventral nerve cord; r i> u ,J,, 



, , parts of an animals body 



w, wing; /, leg; m, muscles. r J 



(Much enlarged; after Graber.) COmpOSC its skeleton, 



W. 



v.n.'c. 



