260 THE BODY AT WORK 



The feature of this type of muscle is its transverse striation, 

 almost mathematically regular. Commonly striated muscle is 

 spoken of as " voluntary," because, for the most part, it is 

 under the control of the Will ; but the term, in so far as it 

 implies a connection between structure and mode of actuation, 

 is misleading. Transverse striation is evidence of capacity for 

 rapid action. The muscles which the Will directs exhibit 

 promptitude ; but striated muscle, which is not under the 

 direction of the Will, is found in certain situations e.g., the 

 upper part of the oesophagus. Conversely, many animals can 

 voluntarily call into action muscle which is not striped. A 

 turkey erects its feathers by setting in motion little groups of 

 " plain " fibres, which pull on elastic tendons attached to the 

 tips of the buried ends of their shafts. Plain muscle contracts 

 less promptly and relaxes more slowly than the striped variety. 

 Cardiac muscle is quicker in acting than plain, but does not 

 hold the contraction so long. 



All striped muscle is not equally rapid. Two varieties are 

 distinguishable : " white fibres," which respond suddenly to a 

 single stimulus and quickly relax ;" red fibres," which respond 

 in a more leisurely way, but remain contracted longer. In 

 some muscles these two types of fibre are intermixed. Others 

 are wholly red or wholly white. Everyone is familiar with the 

 contrast which the white flesh of a turkey or of the domestic 

 fowl presents to the red flesh of game-birds and birds of prey. 

 In the breast of a blackcock a sheet of white muscle overlies a 

 mass of red. When the bird is cooked the difference in colour 

 is strongly marked. Of the two muscles which, in a rabbit, 

 correspond to our muscles of the calf, the superficial, gastroc- 

 nemius, is white ; the deeper, soleus, red. The former acts 

 over both knee and ankle joints ; the latter over the ankle 

 only. The muscle which, acting over a longer range, has to 

 contract more quickly is white ; the shorter, more slowly acting 

 muscle is red. Experiment shows that red and white muscles 

 are distinguished by a difference in the promptitude with which 

 they respond to an electric current. It shows, too, that the 

 white muscle is exhausted sooner than the red. It cannot give 

 so many successive responses to stimulation without a rest. 

 We shall find, when we are considering the minute structure of 

 striped muscle, a difference between its two varieties which 



