MUSCULAR ENERGY 45 



such as the shoulder, a larger number of muscles is 

 necessary to insure exact and powerful control. In gen- 

 eral, muscles are arranged in pairs, so that each muscle 

 as it lies on one side of the joint has on the other side 

 an opposing muscle ready to pull the bone in the oppo- 

 site direction. In many cases where powerful action 

 is required, the principal pair of opposed muscles is 

 reinforced by assisting muscles. Sometimes, there are 

 as many as four or five muscles on one or both sides 

 of a joint, as in the knee. 1 



Production of energy. During muscular activity, 

 animals give off in their breath an increased amount of 

 carbon dioxide/ 2 which is produced by tho union of the 

 oxygen that they breathe with the carbon in the food 

 that they eat. This union is a form of combustion, or 

 oxidation, and like all combustion sets free energy, which 

 in the case of the muscles is partly muscular energy 

 and partly heat. In addition to the carbon, other food 

 materials are brought to the muscles by the blood and 

 are oxidized by them for the production of energy. 3 

 The muscle is able to convert one-third of the total 

 energy developed by the combustion into a definite pull 

 upon the bones, whereas the best steam engines are able 

 to convert only one-sixth of the total force produced by 

 the combustion of their fuel into effective work. 



Nerve control. In order that a muscle may develop 

 energy, its cells must be stimulated by their nerves. 

 When the main nerve leading to a muscle is injured or 

 cut, the muscle is unable to work, it becomes soft and 



1 On the front of the thigh, one very large muscle extends the 

 leg; at the back, five much smaller muscles flex it. 



* Carbon dioxide (CO 2 ) equals 1 part of carbon to 2 of oxygeii. 

 (See Appendix A, p. 316.) 



* Each cell of the muscle acts as a separate little engine for the 

 generation of this power. 



