This muscular sense is sometimes called the sense of motion; and rightly, because it is a feeling of muscular action; not a feeling of extraneous bodies, but a feeling of what passes within ourselves, as far as such feeling is permitted (many internal operations go on without our consciousness)--a feeling of what we do, according to our will, and the permission of God Almighty. Does the uninstructed man, who looks upon and admires a lovely landscape, think about the inverted picture on his retina? No. Does the athlete who lifts a great weight think of the biceps, or the deltoid, or the pectoral muscles? No. But does he not feel, while he strives, while he struggles, while he runs, an internal consciousness of action totally distinct from touch?
This perception is termed by sir Charles Bell the "muscular sense." He thus illustrates his idea: "When a blind man, or a man with his eyes shut, stands upright, neither leaning upon nor touching aught, by what means is it that he maintains the erect position? The symmetry of his body is not the cause. The statue of the finest proportions must be soldered to its pedestal, or the wind will cast it down. How is it, then, that a man sustains the perpendicular posture, or inclines in a due degree towards the winds that blow upon him? It is obvious, that he has a sense by which he knows the inclination of his body, and that he has a ready aptitude to adjust it, and to correct any deviation from the perpendicular. What sense, then, is this? for he touches nothing, and sees nothing; there is no organ of sense hitherto observed which can serve him, or in any degree aid him. Is it not that sense, which is exhibited so early in the infant in the fear of falling? Is it not the full development of that property which was early shown in the struggle of the infant, while it lay in the nurse's arms? It can only be by the adjustment of muscles that the limbs are stiffened, the body firmly balanced and kept erect. There is no other source of knowledge but a sense of the degree of exertion in his muscular frame, by which a man can know the position of his body and limbs, while he has no point of vision to direct his efforts, or the contact of any external body. In truth, we stand by so fine an exercise of this power, and the muscles are, from habit, directed with so much precision and with an effort so slight, that we do not know how we stand. But if we attempt to walk on a narrow ledge, or stand in a situation where we are in danger of falling, or rest on one foot, we become then subject to apprehension; the actions of the muscles are, as it were, magnified, and demonstrative of the degree in which they are excited.