162 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



Tetanus. When a frog's muscle is stimulated by a single electric shock, it 

 contracts and relaxes again within a tenth of a second. Usually muscles do not 

 move so fast. This is because practically all contractions of voluntary muscles 

 are tetanic, the results of rapidly repeated stimuli, which maintain their con- 

 traction. When you carry a box of eggs your arm holds them steadily, not by 

 jerks. In human skeletal muscle tetanus results from stimuli entering the mus- 

 cle cells at the rate of 40 to 60 per second. They are so close together that the 

 resulting reactions blend into one. This shift from jerk to blend resembles that 

 of moving pictures, the separate pictures are shifted so rapidly that they look 

 continuous. Muscles may tremble and pictures will vibrate when the respective 

 movements are not sure and rapid. 



Production of Heat. Muscles are the greatest living heat-producers. Jumping 

 rope increases the body's outgo of heat but thinking (except as it may involve 

 the muscles) does not, much as it may seem to do so. The heat liberated by 

 muscular action is an extremely important catalyzer which hastens chemical 

 reactions throughout the body. Even the fluffing out of feathers and fur pro- 

 vides extra heat because of the contraction of involuntary muscles in the skin. 

 Honeybees can raise the temperature of their hives a few degrees by the mus- 

 cular exercise of vibrating their wings. 



Muscular Action. The energy for muscular action is freed by oxidation of 

 food. Muscle cells hold a store of food, principally the carbohydrates, glyco- 

 gen made from the glucose brought to them by the blood. How does the 

 chemical energy in the food become the energy of motion in a particular kind 

 of muscle? Although this is only partly known, a great many things have been 

 learned about the minute structure of muscle and the chemical and physical 

 changes which occur in it. A great deal has been discovered through observa- 

 tion and experiment on living muscle, commonly the calf muscle of the leg 

 (gastrocnemius) removed from freshly killed frogs. Organic compounds, such 

 as adenosine triphosphate, and glycogen, which muscle contains, are ready to 

 break down and liberate energy whenever conditions allow it. 



It was long ago discovered that during contraction muscles change their 

 shape but not their size. In one of his excellent experiments the naturalist, Jan 

 Swammerdam (1637-1680), placed a muscle in a container of water attached 

 to a fine capillary tube in which the water line was visible. Then he watched 

 the line while the muscle contracted and wrote, "I must confess that the drop 

 of water sinks so little that I can scarcely observe it." A recent and significant 

 observation of muscular activity is that it may take place in the entire absence 

 of oxygen and without producing carbon dioxide. This means that contraction 

 is not the usual oxidative process, but has not proved that contraction of mus- 

 cle is independent of oxidation. 



Chemical changes occur during muscular action and recovery. One unit of 

 any muscular action consists of a latent period following stimulation, a con- 



