78 A COURSE ON ZOOLOGY. 



changes of position of the body in space. These changes 

 are produced according to the general laws of mechanics ; 

 they result from the action of certain forces on various 

 systems of cords and levers. In animal mechanics the 

 bones represent levers, the muscles are cords, and the 



FIG. 42. 



DIAGRAM OF THE THIRD ORDER OF i EVER. 1, the bone of the arm above the 

 elbow ; 2, one of the bones below the elbow; 3, the muscle that bends the 

 elbow ; this muscle is united, by a tendon, to the bone below the elbow (4) ; 

 at the other extremity, to the bone above the elbow (5) ; 6, the muscle that 

 extends the elbow ; 7, its attachment to the point of the elbow ; 8, a weight 

 in the hand to be raised. The central part of the muscle 3 contracts, and 

 its two ends are brought nearer together. The bones below the elbow are 

 brought to the lines shown by 9, 10, 11. The weight is raised in the direc- 

 tion of the curved line. When the muscle 6 contracts, the muscle 3 relaxes, 

 and the forearm is extended. 



nervous system by its motor fibres acts as the cause of 

 the movements. 



The muscles are composed for the most part of a 

 special tissue having contractile power, called muscu- 

 lar tissue, and their contractile activity is excited by 

 impulses from the nervous system. Movements result 

 from the change of position of the long levers to which 



