64 CONSCIOUS NERVOUS OPERATIONS 



lasting for some time are also produced in the muscle 

 by its contraction. These are shown by the use of a 

 delicate galvanometer. 



76. Another important effect upon itself of a muscle's 

 contraction is what we call fatigue of the muscles, that is, 

 a lack of readiness to respond to stimulus. This is due 

 to the using up of the material in the muscles which was 

 available for the production of energy, and still more to 

 the accumulation of waste matter the product of the 

 activity of the muscles. Experiments have shown that it 

 is not the muscle itself which first becomes too much 

 fatigued for contraction. Nor is the seat of fatigue in 

 the nerve, but in the end plate within the muscle cell. The 

 fatigue is relieved by even a brief rest, and such relief is 

 absolutely necessary to the health of the muscles. Even 

 the muscles of the heart, that organ which works cease- 

 lessly from the beginning of life to death, have a period 

 of rest after each beat. 



77. External Effects of Muscular Contraction and Relaxation. 

 The purpose of muscular contraction is the production 

 of motion. The contraction and relaxation of the muscular 

 walls of the heart keep the blood in constant movement ; 

 the various other vital processes are also dependent upon 

 more or less constant motion in the tissues and organs of 

 the body, and all our outward activities are likewise the 

 results of the shortening and lengthening of the innumer- 

 able strands of muscular tissue. 



Muscular power, or the amount of force which a mus- 

 cle can supply, varies with its health and vigor, and with 

 its form. The thickest muscles can lift the heaviest 

 load. Those having the longest strands can move a 

 weight the greatest distance. Hence the human body 

 possesses both long, slender muscles, and short, stout ones, 



