36 THE MUSCLES 



The muscles which produce extension are more powenul than 

 those opposite to them. Muscles which flex are called flexors; 

 those which straighten are called extensors. Other muscles 

 draw the limbs away from the axis of the body, and are called 

 abductors. These are opposed by muscles whose action brings 

 the limb back toward the axes of the body ; such are, in con- 

 sequence, called adductors. In general, the skeletal muscles 

 are paired in the manner just described ; when one contracts, 

 its mate must relax, and vice versa. This arrangement is 

 commonly called antagonistic. 



7. The muscles are also distinguished as the voluntary and 

 involuntary muscles, according as they are, or are not, under 

 the control of the will. The heart is an example of the invol- 

 untary variety. We cannot change its action in the least by 

 an effort of the will. When we sleep, and the will ceases to 

 act, the heart continues to beat without cessation. The volun- 

 tary muscles, on the other hand, are such as are used only 

 when we wish or will to use them as the muscles of the hand 

 or arm (Figs. 13 and 14). (Read Note 1.) 



8. The Tendons. Tendons, or sinews, are the extremities 

 of muscles, and are firmly fastened upon the bone. They are 

 very strong, and of a silvery whiteness. They may be felt just 

 beneath the skin, in certain parts of the body, when the muscles 

 are being used, as at the bend of the elbow or knee. The 

 largest tendon of the body is that which is inserted into 

 the heel, called the tendon of Achilles, after the hero of the 

 Grecian poet, the fable relating that it was at this point that 

 he received his death-wound, no other part of his body being 



1. The Perfection of the Human Hand. " Gordy counts thirty-four 

 distinct movements of the hand, and if we include the combinations of 

 these different movements, we shall reach a much higher number. Prop- 

 erly speaking, the hand belongs to man alone, and its form does not per- 

 mit us to consider it an organ of locomotion, as is the case with certain 

 animals most closely resembling man. Nothing gives a more complete 

 idea of the perfection of the mechanism of the hand than the execution of 

 instrumental music. Examine an artist while he plays the violin. His 

 fingers rest upon the strings so as to leave them exactly of the length 



7. Describe the voluntary and involuntary muscles, and give examples of each. 8. De- 

 scribe the tendons. Mention one. 



