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, gastrocnemius, 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 we can correlate with their different modes of action. All human muscles belong to the red kind.