The loss of the sense of touch in the skin is not, however, the sole or even chief reason that the ataxic man cannot walk. I once saw a patient who could march very well by night or by day, although the skin of his legs was so dead that a live coal would scarcely awaken it. The sense that is wanting in locomotor ataxia is that form of general sensibility which is known as the _muscular sense_; that is, that sense or feeling which tells the lower brain exactly how much a muscle is contracted, and when to urge it more, when to let it relax. This is evidently a guiding sense, of whose action we are barely conscious, but of whose existence there is no doubt. We do not feel a muscle contract ordinarily, but we do so when attention is directed to a muscle at work. Thus we judge of the weight of a body by the amount of force which our muscles must put forth to lift it. Any one can satisfy himself by a very simple experiment that the judgment of weight is not founded upon pressure on the skin. Lay the hand upon the table, place the heavy body upon it and try to judge of the weight. Again, whenever we endeavor to estimate accurately how heavy a body is, we raise it again and again into the air, evidently that we may feel the muscles contract again and again, and by repeated efforts judge how much of force they put forth. The phenomena of ataxia are of great importance as corroborating the experimental proofs spoken of a few pages back, which show that walking is automatic and not simply performed by a conscious power of the will. Thus the ataxic may be able to walk with his eyes open, when he cannot even balance himself erect with his eyes shut. If the conscious will really did by a direct effort produce walking, it should be able to control the movements by itself, and the ataxic man should be able to walk although impulses from the exterior of the body no longer reached his lower brain. The man who turns a cog-wheel himself is independent of external power, but the man who directs a clockwork moved by a spring can only push back the stop and release the spring: if the spring be broken he is powerless. The impulses from the exterior are the springs of locomotion, and all the will can do is to withdraw the checks and call the machinery of walking into action. When the external impulses are no longer transmitted to the lower brain walking becomes impossible.