Readers versed in psychological literature will have missed, in our account thus far, the usual invocation of 'the muscular sense.' This word is used with extreme vagueness to cover all resident sensations, whether of motion or position, in our members, and even to designate the supposed feeling of efferent discharge from the brain. We shall later see good reason to deny the existence of the latter feeling. We have accounted for the better part at least of the resident feelings of motion in limbs by the sensibility of the articular surfaces. The skin and ligaments also must have feelings awakened as they are stretched or squeezed in flexion or extension. And I am inclined to think that _the sensations of our contracting muscles themselves probably play as small a part in building up our exact knowledge of space as any class of sensations which we possess._ The muscles, indeed, play an all-important part, but it is through the remote effect of their contractions on other sensitive parts, not through their own resident sensations being aroused. In other words, _muscular contraction is only indirectly instrumental, in giving us space-perceptions, by its effects on surfaces._ In skin and retina it produces a motion of the stimulus upon the surface; in joints it produces a motion of the surfaces upon each other--such motion being by far the most delicate manner of exciting the surfaces in question. One is tempted to doubt whether the muscular sensibility as such plays even a subordinate part as _sign_ of these more immediately geometrical perceptions which are so uniformly associated with it as effects of the contraction objectively viewed. For this opinion many reasons can be assigned. First, it seems _a priori_ improbable that such organs as muscles should give us feelings whose variations bear any exact proportion to the spaces traversed when they contract. As G. E. Müller says,[197] their sensory nerves must be excited either chemically or by mechanical compression whilst the contractions last, and in neither case can the excitement be proportionate to the position into which the limb is thrown. The chemical state of the muscle depends on the _previous_ work more than on the actually present contraction; and the internal pressure of it depends on the resistance offered more than on the shortening attained. _The intrinsic muscular sensations are likely therefore to be merely those of massive strain or fatigue, and to carry no accurate discrimination with them of lengths of path moved through._ Empirically we find this probability confirmed by many facts. The judicious A. W. Volkmann observes[198] that: