THE MACHINERY OF COORDINATED MOVEMENTS. 647 



existing ones, so modify the flow of afferent impulses into the machinery of 

 coordination as to throw that machinery out of gear. 



556. We have dwelt on these phenomena of the semicircular canals 

 because they illustrate in a striking manner the important part played by 

 afferent impulses in the coordination of movements. We saw reason to 

 think ( 502) that even in an ordinary reflex movement carried out by the 

 spinal cord or by a portion of the cord, afferent impulses, other than those 

 which excite the movement are at work, determining such coordination as 

 is present. In such a case the coordinating afferent impulses are relatively 

 simple in character and start chiefly at all events in the muscles concerned. 

 In an animal possessing the lower parts of the brain, though deprived of the 

 cerebral hemispheres, the coordinating afferent impulses, in accordance with 

 the greater diversity and complexity of the movements which the animal is 

 able to execute, are far more potent and varied. Besides afferent impulses 

 from the muscles, forming the basis of what we have called the muscular 

 sense, afferent impulses from the skin, forming the basis of the sense of touch 

 in the wide meaning of that word, other afferent impulses of obscure cha- 

 racter from the viscera and various tissues, and the peculiar afferent ampullar 

 impulses of which we have just spoken, important special afferent impulses 

 borne along the nerves of sight and hearing come into play. The frog, the 

 bird, and even the mammal, deprived of the cerebral hemispheres, though it 

 may show little signs or none at all of having a distinct volition, is, as we 

 have urged, indubitably affected by visual and auditory impressions, and 

 whether we admit or not that such an animal can rightly be spoken of as 

 being conscious, we cannot resist the conclusion that afferent impulses started 

 in its retina or internal ear produce in its central nervous system changes 

 similar to those which in a conscious animal form the basis of visual and 

 auditory sensations, and we must either call these changes sensations or find 

 for them some new word. Whatever we call them, and whether conscious- 

 ness is distinctly involved in them or not, they obviously play an important 

 part as factors of the coordination of movements. Indeed, when we appeal 

 to the experience of ourselves in possession of consciousness, we find that 

 though various sensations clearly enter into the coordination of our move- 

 ments, we carry out movements thus coordinated without being distinctly 

 aware of these coordinating factors. In every movement which we make 

 the coordination of the movement is dependent on the impulses or influences 

 which form the basis of the muscular sense, yet we are not distinctly con- 

 scious of these impulses ; it is only, as we shall see, by special analysis that 

 we come to the conclusion that we do possess what we shall call a muscular 

 sense. So again, taking the matter from a somewhat different point of view, 

 many of our movements, markedly, as we shall see, those of the eyeballs, are 

 coordinated by visual sensations, and when we sing or when we dance to 

 music our movements are coordinated by the help of sensations of sound. In 

 these cases distinct sensations in the ordinary sense of the word intervene; 

 if we cannot see or cannot hear, the movement fails or is imperfect ; yet 

 even in these cases we are not directly conscious of the sensations as coordi- 

 nating factors; it needs careful analysis to prove that the success of the 

 movement is really dependent on the sound or on the sight. These and 

 other facts suggest the view that the point at which the various afferent im- 

 pulses which form the basis of the sensations of a conscious individual enter 

 into the coordination mechanism is or may be some way short of the stage 

 at which the complete conversion of the impulse into a perfect sensation 

 takes place. The events which constitute what we may call visual impulses, 

 as these leave the retina to sweep along the optic nerve are, we must admit, 

 very different from those which in the appropriate parts of the brain con- 



