EFFICIENCY OF MUSCLES 131 



about 40 miles an hour but when pressed can lly faster. 

 Swifts which have been carefully timed co\Tr a distance of 

 171-4 to 200 miles an hour. 



In the higher animals locomotion is brought about by the 

 contraction of muscles. The muscle becomes shorter and 

 thicker. It will contract till it is from 65 to 85 per cent, shorter 

 than before. But its volume scarcely changes. 



From the point of view of doing work, the muscle is a 

 very efficient instrument. An ordinary locomotive loses 

 about 96 per cent, of its available energy in the form of heat, 

 and only 4 per cent, is left to represent the work done. Even 

 in the very best triple expansion steam-engines the work done 

 rises but to 12-5 per cent, of the total available energy. In 

 the vertebrates, muscles form by far the larger bulk of all 

 tissues, some 45 per cent, of the whole body-weight. Muscles 

 are the chief source of the heat of the body, and the heat of 

 the muscles is increased when they contract. This heat is 

 furthermore not wasted as it is in an engine. It keeps the 

 body at a temperature necessary for life and health. In man 

 from 20 to 28 per cent, of the energy liberated during the 

 contraction of the muscles appears as work — five to seven 

 times as much as in a locomotive. Muscles absorb more oxygen 

 and release more carbon dioxide than any other tissue. A 

 hundred grammes of muscle will take up 5-68 cc. of oxygen 

 and give out 5-08 cc. of carbon dioxide per minute, whereas 

 the same amount of the next most active organ, the brain, 

 takes up but 4-58 cc. of oxygen and releases 4-28 cc. of carbon 

 dioxide. Certain drugs will put the action of muscles out of 

 control, and when that is done the temperature of the body 

 drops. 



Normal muscular contraction is an anaerobic process — as 

 opposed to relaxation, which involves the oxidation of lactic 

 acid (which has been produced from the glycogen). It is 

 conceivable therefore that if proper mechanisms, i.e. not 

 requiring oxidation, were provided for the removal of tiu' 

 lactic acid which provokes a muscle to contract, movement 

 could occur indefinitely in the absence of oxygen. The need 

 for oxygen in higher animals seems to be concerned more 

 intimately with the nervous processes. 



9-a 



