242 ELEMENTS OF BIOLOGY 



ferences between these three types (Fig. 167). Muscles are also 

 inaccurately classified as voluntary and involuntary, the striated 

 muscles being regarded as voluntary and the smooth and cardiac 

 muscles as involuntary. Since many striated muscles are not under 

 voluntary control and since all may occasionally contract involun- 

 tarily, the classification is of limited value. 



Energy and Fatigue in Muscles. The contraction of a mus- 

 cle cell is essentially a change of shape, the long dimension being 

 shortened and the diameter being increased. By the shortening of 

 the long dimension work is done. This requires energy. The energy 

 of muscle contraction is known to come from the breakdown of 

 muscle sugar, glycogen, into lactic acid with the release of energy in 

 the form of heat. It is known that with prolonged exercise and the 

 reduction of the amount of carbohydrate available, proteins and 

 fats may also furnish the energy. Some of the lactic acid that is 

 produced (about one-fourth) is oxidized into water and carbon diox- 

 ide; the remainder is converted back to glycogen. The relation 

 between the chemical events that occur in muscle contraction and 

 the actual mechanical shortening of the cell has not yet been made 

 clear. The lactic acid that is recovered as glycogen after the con- 

 traction is available for the energy of succeeding contractions. Re- 

 cent work has indicated that the fatigue of a muscle is due to two 

 factors, the accumulation of lactic acid in the cells and the disap- 

 pearance of the glycogen. 



Muscle Work Units. A striated muscle cell is a multinuclear, 

 long, cylindrical fibre. Fibres are bound together by connective 

 tissue as bundles and the bundles form the separate muscles. Mus- 

 cles occur in antagonistic sets and work is done by the contraction 

 of one set accompanied by the relaxation of the other. For example, 

 the forearm is flexed by the contraction of the muscles on its an- 

 terior face accompanied by the relaxation of the muscles on the 

 back of the arm; when the arm is extended the contracted muscles 

 relax and the arm is pulled straight by the contraction of the 



