48 THE INDIVIDUAL ORGANISM 



and moved a part of the body, it requires the contraction of the antago- 

 nistic muscle or muscles to restore the skeletal parts to their original 

 position and stretch out the relaxed muscle. The biceps pulls the forearm 

 up and bends the elbow; the triceps on the back of the upper arm is the 

 chief antagonistic muscle which pulls the forearm down, straightens the 

 elbow, and stretches out the biceps. In this it may be aided by gravity. 



The body muscles. The human body contains some 700 skeletal 

 muscles, integrated into a muscular system and constructed upon the 

 plan of opposing and antagonistic sets of muscles. Any movement that 

 we can make requires the contraction of one or more (and usually several 

 or many) muscles. Most muscles take part in a considerable variety of 

 movements, the variations depending upon what other muscles cooperate 

 to produce a given motion. In view of the great versatility of movement 

 of the human body it is easy to understand why it contains several 

 hundred separate muscles, and why the total skeletal muscle tissue com- 

 prises much more than half of the total body weight. 



In addition to causing movement the skeletal muscles have another 

 important function — that of producing heat. Indeed, heat production is a 

 nonavoidable result of muscular activity. Some 70 to 80 per cent of the 

 chemical energy that is released in the muscles is transformed into heat, 

 and only 20 to 30 per cent goes into contractile energy. This is about the 

 same percentage of efficiency as that of a good steam or internal combus- 

 tion engine. 



Since our bodies function at a more or less constant temperature of 

 about 98 to 99°F., 1 enough heat to maintain this temperature is utilized 

 and may be regarded as a useful product of muscular action. In most 

 climates, however, even moderate muscular exertion produces an excess 

 of heat, which must be eliminated as a waste product. Occasionally, when 

 our surroundings are cold and our muscular movements have been slight, 

 we shiver; under these circumstances it is the heat produced by muscular 

 contraction that is utilized, and movement (shivering) is the waste 

 product. 



THE INTEGUMENTARY SYSTEM, OR SKIN 



Structurally the skin would scarcely qualify as a typical system, ac- 

 cording to our definition of a system as a functional unit composed of 

 organs. Functionally, however, it has a much better claim to be regarded 

 as a system. Its functions are many and diverse and are grouped into the 

 responsibility of a single system not because they are physiologically 

 related but because they all have to do with the surface of the body. 



1 The normal mouth temperature, marked on the clinical thermometer, is 98.6°F., 

 but temperature shows a regular daily cycle of rise and fall, and internal temperatures 

 are somewhat higher than those in the mouth. 



