TISSUES AND ORGANS 21 



14. The following table shows this classification : 



1. Tissues which have to do with liberating j Muscular 

 energy Master Tissues j Nervous 



15. The Master Tissues. Muscular tissue liberates 

 energy which takes the form of motion attended by some 

 measure of heat. But the changes in muscular tissue 

 by which energy is liberated are guided, regulated, and 

 adapted to the purposes of human life by means of nervous 

 tissue; that is, the muscles are the instrument, but an 

 instrument motionless and useless until the nerves supply 

 the impulse which sets the muscles at work. A muscle 

 develops energy under the action of nervous stimulus con- 

 veyed to the muscle cell through nervous tissue. Energy 

 is set free in nervous tissue when the nervous organ 

 is stimulated by the influence adapted to it. The nerves 

 carry the impulse to the muscle, and energy is liber- 

 ated as motion and heat. Muscular and nervous tissues 

 possess irritability, that is, they respond to stimulus, in 



