whole, to show (1) that muscular discrimination involves an afferent element, (2) that this is distinct from ordinary tactile sensibility, and, though commonly including skin sensations, e.g., those due to the stretchings and overlappings of the skin which accompany movements of the limbs, involves also the stimulation of nervefibres running to the tendons and to the surface of the joints, and possibly also of these fibres which are supposed to end in the muscles themselves. There is, however, no agreement as yet respecting the exact parts played by each of these varieties of afferent sensation. Whether these afferent elements are the sole constituents, or whether an efferent element, vis., a sensation of innervation immediately attending the process of central innervation, is not also involved, is still a matter of keen dispute, and cannot as yet be said to be determined by the known facts.

In this condition of things it seems better that the psychologist should take up the conciliatory position that our muscular sensations are at once the correlative of efferent and of afferent processes. The existence of separate motor structures and organs in the brain itself suggests that the process of central discharge is one part of the neural substrate of our psychical life.1 Not only so, the fundamental distinction between the process of afferent stimulation and of efferent discharge supplies, as already pointed out, an adequate physiological counterpart for the unquestioned psychological contrast (probably the deepest-reaching within the circle of our experience) between passive and active consciousness. Hence the psychologist does well not to abandon the idea of a consciousness attached to the outgoing current till some crucial experiment not yet devised shows this to be non-existent.2

§ 30. Varieties of Muscular Sensations. The action of the voluntary muscles gives rise to a considerable variety of sensational experiences. This variety of sensation is connected partly with differences in the muscular and other peripheral structures involved, partly with differences in the mode of action of these structures.

(a) Differences depending on particular Muscles, etc., engaged. To begin with then, it is evident that since our (voluntary) muscular system, unlike a special sense-organ, extends over the whole area of the body and certain of its cavities, and is made up of very unlike organs or structures, differences of peripheral

1 It must be conceded, however, that some who adopt the theory that muscular sensation is wholly afferent deny the existence of separate motor central structures. This helps their theory, but is not generally supported by anatomists.

2 Among recent discussions of the muscular sense the best are Bain, The Senses and the Intellect, p. 76 ff.; Wundt, op. cit., i. p. 397 ff.; Bastian, The Brain as an Organ of Mind, Appendix; Ferrier, The Functions of the Brain, p. 62 ff. and p. 382 ff.; Beaunis, Les Sensations internes, p. 61 ff.; Mack, Die Bewegungs-Empfindungen; T. Lipps, Psychologische Studien, p. 6 ff.; Münsterberg, Die Willenshandlung, p. 78 ff., and Beiträge zur exper. Psychol. i. p. 152 ff.; and James, Principles of Psychology, ii, p. 189 ff. and p. 494 ff. The a priori psychological argument in favour of an efferent nervous substratum is well put by Fouillée, Revue Philosophique, xxviii. p. 561 f.

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structure will produce differences in the psychical concomitant. A difference of calibre, as between the muscles of the leg and of the fingers, will affect the quantity of the muscular sensation, making it more or less massive or extensive; not only so, difference in the attachments of the muscles and adjacent tissues will modify the quality of the accompanying sensation in various ways. Thus the psychical correlative of the action of the muscles of a limb will be "coloured" by the peculiar articular sensations connected with the pressure of the joint-surfaces, and movements of the same, an element which is wanting in the case of the ocular muscles, and the vocal muscles. So the psychical concomitant of the action of the muscles of the jaw will be differentiated by the presence of the peculiar element of reciprocal pressure of the teeth, etc.

These qualitative differences would, it is evident, serve to differentiate the sensations corresponding to the action of particular groups of muscles by quasilocal differences. It is a point of dispute whether the mere fact of the several muscles having each its own motor and probably also sensory elements would serve to give distinctness to the resulting sensations. This position might be maintained on the ground already indicated, viz., that to (numerical) difference of nervous structure engaged there corresponds in general some difference in physical concomitant. It seems, moreover, to be involved in the fact that we have a more massive or bulky sensation when a large number of muscular structures is engaged.

This postulate might be adopted on either of the views of the nature of the nervous process in muscular experience set forth above, viz., that the process is an efferent or an afferent current. It is fairly certain, however, that the local distinctions among our muscular sensations always involve as a principle factor other elements, viz., the analogues of "local" differences in the sensations of articular surface, skin, etc., which enter into and colour the whole muscular experience.2

(b) Differences Arising from Mode of Muscular Action. We may now pass to the differences of muscular experience connected with dissimilarities in the mode of action of the muscles engaged. Here we may confine ourselves to those groups of

1 It is to be noted that so far as we know a muscle is never at work in isolation. All muscular action is a resultant of the functional activities of groups of muscles, as a flexor and its antagonist extensor.

2 Wundt (op. cit., p. 299), the most important of the latest exponents and defenders of the 'efferent current' theory, appears to allow that the mere fact of the process of innervation discharging into this or that particular motor channel would not affect the character of the accompanying 'feeling,' i.e., sensation of innervation.

muscles which are of chief importance as a source of knowledge, viz., those by which our limbs are moved.

The most obvious contrast in the domain of muscular experience is that between action issuing in movement and action not issuing in movement. Since movement is the normal result of muscular contraction in the absence of all counteractives, it would seem proper to begin with an account of this. There will, however, be a certain advantage in proceeding in another way. We may set out with the comparatively simple case of a momentary experience, viz., that answering to a particular position of the limb. We may then consider the more prolonged experience of movement itself, and finally take up the complicated case which arises when movement is impeded by the presence of an obstacle. Thus we have (1) Muscular Experience without Movement: Sensations of Position; (2) Experience of Movement; and (3) Experience of Impeded Movement.

These types of motor experience are not absolutely marked off one from another. Since any particular position of a limb is taken up by some movement it follows that the sense of position is connected in the closest way with the experience of movement. On the other side, the active maintenance of a position, as in keeping the arms outstretched, or the head erect, is carried out by an opposed action of certain muscles and their antagonists, so that the experience has something in common with that of impeded movement. Again, even free movement is apt, when we are fatigued, to be attended by a sense of obstacle in the shape of the weight of the limb itself. The transition from this experience to the full experience of impeded movement is supplied by the loading of the limb, as when we lift a weight.

(b. 1.) Experience without Movement: Sensations of Position. The experience answering to a particular position of the limb may arise either passively or actively. A person may support my outstretched arm, or I may myself hold it out. The former situation, position passively induced, is obviously the exceptional one, at least in later life. It is complicated by the skinsensations of pressure, while, on the other hand, it does not involve the characteristic action of the muscles as made known in active consciousness or sense of exertion. We may then dismiss this case, and confine our attention to the normal experience of actively-induced position.

It follows from what has been said respecting the probable constituents of the muscular sense that varying positions of a

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limb, say the arm, will have varying psychical concomitants. We can infer from the mechanism of muscular action that the relative position and direction of pressure of the articular surfaces, the condition of the tendons and of the muscles themselves, as also that of the enveloping skin, will vary for every successive position as I flex the fore-arm; and these changes will bring on corresponding psychical changes. These differences of positional sensation are of the greatest importance, as we shall see, for the acquisition of a knowledge of the spatial relations of our own body and of our surroundings.

It is important to note that the muscular action corresponding to a particular position of the arm is not a fixed quantity, but varies according to the amount of exertion employed. I can use a comparatively little, or very much, effort in holding out my arm. The explanation of these differences is that given by Münsterberg, viz., that one and the same effect of a muscular action, maintenance of position, or movement, may result from different amounts of muscular work, so long as the ratio between the amounts of work of the muscles and their antagonists remains the same.1

It is to be added that the varying position of our limbs makes itself known by passive experience also. Thus, when on lying down my arm lies lengthwise on the trunk, the position is known mainly by help of the skin-sensations of pressure. Muscular consciousness is, however, not wholly absent here; for it must be remembered that this is supposed to attend all conditions of the muscles, and to vary characteristically for states of relaxation as for those of tension.

(b. 2.) Experience of Movement. In the case of movement we have, it is evident, a prolonged experience, made up of a continuous change or succession of sensational accompaniments. This feature of change is essential and characteristic. Movement is not merely that by which we bring about indirectly changes in our surroundings, e.g., the visible scene, it is itself an experience of change. It is reasonable to suppose that the delicacy of our sense of movement depends on the fineness of our discrimination for these successive sensational differences.

1 See his Beiträge, i. p. 155.

The discriminative delicacy characterising motor sensibility or sense of movement has been estimated in the case of the ocular muscles which bring about movements of convergence. Here it is found to be very great. Thus a movement of the eyes (or the optic axes) through an angle of 68 seconds, answering to a contraction of the inner muscle of the eye-ball amounting to about 004 millimetres, was detected. And a difference in the range of movement, corresponding (on the average) to the fraction, was perceptible. According to some recent experiments of Münsterberg, Weber's law holds good of the least noticeable differences of linear magnitude as appreciated by the sensations of the ocular muscles. See his Beiträge, heft ii. p. 180 f.

In considering these experiences of movement it is important to keep them separate from that spatial interpretation which, as we shall see, comes later. Motor experience, like experience of position, may arise passively, as when another moves my arm, or actively, as when I move my own arm. We will touch on the difference between these two experiences presently, after considering the general characters of movement.

Movement as consciously experienced, as we shall see byand-by, is, along with the original local distinctions of passive sensation, a main source of our knowledge of space. As already suggested, the normal result of the action of our muscles is movement. Position is taken up and altered by movement. It is by movement that we explore the surface of our own bodies, as also environing space and its objects.

In order to explain the genesis of our perceptions of position, distance, etc., by help of motor experience, it seems necessary to assume two presentative characters in our motor experience: (a) that answering to direction of movement, and (b) that answering to range of movement. With respect to the psychical concomitant of direction, it has been already pointed out that the action of one group of muscles will consciously differ from that of another. In this way the movement of the right arm and of the left would affect our consciousness differently. Not only so, movements of the same arm in different directions would, for a similar reason, have different psychical concomitants. Thus the flexing and extending of the fore-arm would differ in consequence of the difference in the order of succession of the sensations attending the changing positions of the articular surfaces, tendons, skin, etc., in the two cases.1

1 Whether the sensations of innervation which we have supposed to be a constituent of the consciousness of movement would differ in these two cases is, as already suggested, doubtful.