136 THE MUSCLE CELLS 



(the nervous system) exists in large measure to suit the muscles. 

 In short, the muscles are the master-tissues of the soma. They 

 are not a manufacturing community but are power users. In 

 another sense the muscles are the servants of the body. By 

 means of them the body fights a war not merely of defence but 

 of aggression against its environment. As civilisation has 

 advanced, man has found it -convenient by means of tools and 

 machines to add to his muscle power. In this way, he has been 

 able to harness and utilise power from sources that could not have 

 been tapped without the use of these machines. Tools and 

 machines are thus as it were extended and detachable limbs. 



Muscle is a transformer of potential energy into kinetic energy, 

 and of course into heat. The source of the energy is, as we have 

 seen, the foodstuffs as broken down by enzymes, i.e. glucose, fat 

 and deaminised amino acids. .There seems to be no doubt as to 

 the means by which the energy is ultimately liberated from these 

 food units. In Chapter III. we saw that the intake of oxygen 

 and the output of carbon dioxide was so intimately connected 

 with the process of energy liberation that we could calculate the 

 amount of energy set free in an organ by measuring these gases. 

 That is, the basis of muscular activity is oxidation, just as the 

 basis of the activity of a steam engine is the oxidation of coal. 

 In Chapter IV. there was a discussion of this, and evidence was 

 produced proving that in all other details the oxidative process 

 in muscle differs from that of coal. What then happens in muscle ? 

 There seems to be three stages in the translation of potential 

 energy of food into muscular work. 



(1) Contraction. The muscle shortens and thickens. During 

 this phase oxygen is not necessary. An excised frog muscle in 

 nitrogen will give a maximal contraction once every five minutes 

 for over two hours before showing signs of fatigue. The output 

 of carbon dioxide is not increased over the amount evolved during 

 rest. The heat evolved is negligible if the muscle is allowed to 

 shorten under certain conditions. If it is made to contract 

 isometrically i.e. against a spring that does not permit of its 

 shortening, the energy developed, that would in ordinary course 

 produce contraction, appears as heat. This heat is directly pro- 

 portional to the length of the muscle fibres. Further, increase 

 of temperature causes a decrease in the tension developed on 

 stimulation, i.e. contraction has a negative temperature co- 

 efficient. A muscle at C. in an experiment of Bernstein's 

 developed a tension of 375 grams. On raising the temperature 



