April i6, 1908] 



NA TURE 



571 



changes, and the more so the more abrupt the change. It 

 is, therefore, changes in the outside world that operate 

 especially as stimuli, though, of course, only changes 

 which have relation to the animal in question. If we 

 regard the mutual relation between the animal and the 

 world at any moment as an equilibrium, then we can say 

 that any change in the world which changes that relation 

 disturbs the equilibrium. 



Take the instance of a child asleep. A thousand 

 agencies of the external world are playing upon it. Upon 

 its skin, for instance, there is the pressure of the child's 

 own weight against the receptors, and there is the pressure 

 of the clothes which cover it ; yet it lies restful. Suppose 

 we touch its foot. That is a change in the external world 

 in relation to the child. The familiar fact is that the foot 

 is drawn up out of harm's way, as it w'ere. The change 

 has acted upon the child as a stimulus to some receptors 

 of the skin. It may be quite unconscious of the touch, 

 for its sleep may be deep. Yet the reflex action has 

 occurred, and has done the appropriate thing. .\ candle 

 may be brought into the room and its light reach the face 

 of the child. That is a change in the outside world in 

 relation to the child. The familiar fact is that the child's 

 head turns from the light. It sees no light, but reflex 

 action averts its face. Or, turning to other forms of life, 

 take a fish quiet in its aquarium. A worm is dropped 

 into the water, and the disturbance of the water reaches 

 the surface of the fish. The fish turns and seizes the 

 morsel. Such a reaction on the part of such a creature 

 is probably wholly reflex. 



The point for us here is, that the changes in the outside 

 world whicli act as stimuli bring about appropriate re- 

 adjustments of the body to the external world, and that 

 in doing so the instruments of readjustment are the 

 skeletal muscles, worked by the nervous system. The 

 child's heart goes on beating, whether the child's foot lies 

 quiet or is moved, whether its face lies this way or lies 

 that; the fish's heart whether the animal's skin was 

 stimulated by fresh commotion in the water, or was not. 

 But with the skeletal muscles it was different. Flexor 

 muscles of the leg, that were relaxed, are by the touch 

 to the foot thrown into action ; muscles which lay relaxed 

 were, when the light came, caused to contract, turning 

 the head away. Muscles of the fish that were inactive 

 were thrown into activity by the new commotion in the 

 water. It is these skeletal muscles, therefore, that the 

 daily thousand changes of the external world so repeatedly 

 and constantly affect in this way or that, and in reflex 

 action it is always the receptors and the nervous system 

 which impel them to react ; and the result is to re-adjust 

 advantageously to the animal its relation to the altering 

 external world. Hence these muscles are called the 

 muscles of external relation. So prominent are these 

 muscles in the everyday work of life that they are the 

 muscles of ordinary parlance. The man in the street is 

 hardly aware that he has in his body any other muscles. 

 These muscles are, through the nervous system, driven 

 by the external world. The world outside drives them by 

 acting on the receptors. It is not surprising, therefore, 

 that this little muscle, removed from the body, and there- 

 fore separated from the nervous system and all its re- 

 ceptors, remains, although still living and able to contract, 

 as functionally inactive — for contraction is its function, 

 ard it does not contract — as if it were already dead. 



Now this muscle, w'hen in the body, was the servant of 

 a thousand masters. It had to contribute to a thousand 

 acts. In a certain sense, it, like the heart, had to do for 

 them all but one thing, inasmuch as it had to pull the 

 limb in one certain direction : and yet its task is a very 

 varied one. It has to pull the limb sometimes far, some- 

 times very slightly, or through all intermediate grades. 

 It has to pull it strongly against great resistance, or 

 weakly, and with all intermediate grades of intensity. We 

 may suppose that in the course of evolution it had become 

 adapted to this scope of purpose. 



.And indeed we find it so. Unlike the heart muscle, 

 this muscle when a strong stimulus is applied contracts 

 strongly, when a weak stimulus, weakly : under a long 

 stimulus it contracts long, under a brief, briefly. The 

 nervous system, in making use of this muscle, wants of 

 it just such varied action as this — now weak, now strong, 



XO. 2007, VOL. 77] 



now bnel, now long, as may be suited to the act required. 

 The little organ is admirably adapted to be the animal's 

 instrument in the world in which it is placed. This muscle 

 has its place in the economy of nature, and into it it fits 

 as a key into the lock for which it has been made. Man's 

 naive view, until somewhat recently, was that the earth 

 and the universe were made to fit him. Was the universe 

 made to suit this little muscle or was this little muscle 

 made to suit the universe? The problem concerning this 

 muscle and that concerning man are, in so far, the same. 

 Surely our answer is that the muscle and the rest of the 

 universe fit each other because they have grown up together 

 — because they are part of one great whole ; they fit just 

 as a lock and key fit because they compose one thing, 

 and it is pointless to ask whether the lock was made to- 

 fit the key or the key the lock. 



The office of the nervous system is to coordinate the 

 activities of the various organs of the body, so that by 

 harmonious arrangement the power and delicacy of the 

 animal's mechanism may be obtained to the full. When 

 reflex action withdraws the foot of a sleeping child, it is 

 not merely one such muscle as this which moves the limb, 

 but many. The limb has many muscles, and even in such' 

 a simple act many and many of them are employed. 



That the act occurs during sleep shows that conscious- 

 ness is not its necessary adjunct. A similar act can be 

 similarly evoked .in an animal when the brain — the seat 

 of consciousness — has been removed. The brain can be 

 removed under deep narcosis of chloroform without any 

 pain or feeling whatsoever. .After that removal the animal 

 is no longer a sentient or conscious thing at all. Then we 

 can study in it the power of reflex action sundered from 

 conscient and sentient life altogether. Then it is that 

 opportunity is given for further reverent analysis of those 

 wonderful and subtle workings of the nervous system 

 which in ourselves are so difficult to unravel for the very 

 reason that their working goes on without appeal to, and' 

 often beyond access of, the conscious self. 



When analysing the muscular action of even so simple 

 a reflex act as that of drawing up the foot, a fact which 

 early meets the observer is that the nervous system treats 

 whole groups of muscles as single mechanisms. In lifting 

 the limb it employs together muscles, not only of one joint 

 of the limb, but of all the joints — knee, hip, ankle, &c. 

 It deals witli all these muscles as if they were but one 

 single machine. If the movement is forcible, it throws 

 them all into strong contraction ; if weak, into weak. In 

 the grading of the reflex action its influence is graded in 

 all these muscles alike. So also the contraction in all of 

 them is timed to begin together, to culminate together, 

 and to desist together. Further, although the movement 

 of this lifting of the limb is mainly flexion at its joints, 

 the reflex accomplishes along with that some interna! 

 rotation of the hip and some abduction of the thigh. Why 

 it should do so we shall see presently. Suffice us for the 

 present that, besides the flexor muscles, the nervous 

 system brings into play, at the same time and' 

 harmoniously with those, two other great groups of 

 muscles, the internal rotators and the abductors. So per- 

 fect is its skill in using the muscles as its instruments that 

 it can deal harmoniously and simultaneously with all these 

 individually complex groups of motor organs as though- 

 they were but one. 



Were we to attempt to produce this movement in the 

 limb experimentally without employing its nervous system, 

 we should have to apply I know not how many stimuli" 

 simultaneously to more than half the muscles of the limb. 

 Not only that, but we should have to grade the stimulation 

 of each of these most accurately to a particular strength. 

 We should also have to arrange that, not only did each 

 stimulus develop its full strength with the right speed, 

 but that each should maintain it for the appropriate time 

 and desist at the right speed and moment, and with pro- 

 portioned intensity. Moreover, in the real reflex act the 

 contraction of this or that muscle is now stressed, now 

 subdued, with a delicacy and accuracy baffling all experi- 

 mental imitation. The coordination in even the simple 

 reflex we are considering may be likened to that exhibited 

 by a vast assemblage of instruments in very perfect 

 orchestration directed by a supremelv capable conductor. 

 But it is more subtle and delicate than that, even in 



