The intrinsic factors in the act of progression in the mammal

Abstract Whilst the act of progression is being performed, the several limbs exhibit rhythmic movements of flexion and of extension. When any limb is in contact with the ground, it extends, and thus serves to propel the animal forwards. At the end of this act the limb is lifted from the ground by a movement of flexion, is carried forward, and finally is again placed upon the ground to repeat the cycle. During these phasic acts the dynamic balance of the neural centres is disturbed by two different kinds of peripheral stimuli. In the first place, the discontinuous contact with the ground, and the synchronous distortion of the skin of the foot—determined by the weight of the animal then carried in part by that limb—produce changes in the activity of exteroceptive end-organs therein embedded, and discontinuous augmentations and diminutions of the stimuli originated in them.