II.
Of the first kind we need say no more. The instance of peristaltic movement illustrates it sufficiently; so we can at once begin a more careful examination of reflex action.
The simplest instance of reflex action may be taken from the schoolroom. If a boy suddenly sticks a pin into an unsuspecting schoolfellow, the latter invariably starts, and frequently lets fall an exclamation also. In this case the presence of an injurious agency is reported to the nearest motor centre, which is in the spinal cord, and this automatically convulses the body, jerking the limb out of danger.
This is reflex movement; the nerve fibre, which conveys an intimation of the injurious influence, is a prolongation, or really two prolongations, of a spinal ganglion cell. ([See Diagram 46.]) The near end of this fibre, which enters the cord, has several branches. Some run a little way up the cord, and some a little way down, so as to communicate with several motor cells; but one branch runs right up the cord, and sends the message on to the brain. Our outraged schoolboy starts a fraction of a second before he is conscious of the pain of being pricked, and this first response is involuntary and unvarying; the sensation, however, is reported to his brain, and the workings of that wonderful organ are less easy to predict. It leads to his taking stock of the aggressor, on the strength of which he decides whether it is safe to attempt a reprisal, and, if so, in what form it will be most effective and least likely to attract the master’s attention. This knotty point settled, the motor cells of the brain send down messages to the motor cells of different parts of the spinal cord, and these in turn set the necessary muscles in motion for delivering a surreptitious kick or aiming a splash of ink, as the case may be. This is voluntary movement.
Diagram 46.—Evolution of a Spinal Ganglion Cell.
Diagram 47.—Scheme of the Central Nervous System.
→ shows the path taken by an impulse in reflex action.
↣ shows the path for a voluntary action.
The difference between reflex and voluntary movement is, as may be seen from the above instances, very much a matter of degree; but we had better leave a comparison between them, and any discussion as to the extent to which the manifestations of consciousness are automatic, until we have finished describing reflex movement, and set forth the little we know about voluntary movement.
Time and space forbid a complete list of reflex movements. The following are, however, a few typical examples of how the body is automatically made to perform such acts as are necessary, and of how such as do not require deliberation are brought about without taxing the intellect.
A reflex action which is unpleasantly familiar is the cough, also the somewhat similar phenomenon of the sneeze. In this case, a foreign body which obstructs the windpipe, or causes irritation to the membrane lining the nose, is, on being reported at the spinal cord, incontinently blown out by an explosive blast of air from the lungs.
An organ which is very important, and at the same time very sensitive—viz., the eye—has many protective reflexes. The external surface of the eye is covered by a very delicate membrane, which must be kept moist and scrupulously clean. Whenever this membrane gets in the least dry, or any dust falls on it, the eyelids are closed for a moment, thereby bathing it with the secretion of the tear glands. Few people are aware, I think, that they blink their eyes on an average twice every minute. The eyes are also closed quite involuntarily by a reflex when any danger threatens them—for instance, a sudden dazzling light, a strong wind, or a blow aimed at the face; and if any foreign substance—say a fly—does get into one of them, the secretion of the tear glands is enormously increased to wash it out.
The size of the pupil, again, is quite involuntarily, i.e., reflexly, altered in proportion to the strength of the light.
Reflex actions are, however, by no means only protective. The act of swallowing is reflex. So is the secretion of the digestive glands when the lining membranes of the stomach are stimulated by the presence of food. The very act of standing depends on the reflex principle, the tendency of the body to collapse and fall being unconsciously perceived and corrected by the spinal cord. Walking is also a reflex action. It may be objected that we think about walking, and do so with intention; but it is of common experience that we can walk along ‘thinking of something else,’ and the way in which an intellectual though absent-minded man will run into people, charge lamp-posts, trip over steps, and tread upon dogs, is sufficient to absolve the organ of thought and intention from any share in the performance.
The blood-pressure is also automatically regulated, both the diameter of the bloodvessels and the frequency of the heart-beat being under reflex control; and we may, as a final instance of reflex action, describe one of Nature’s most perfect and merciful contrivances—fainting. Suppose a man receives a severe wound—say, has his hand struck off by a sword—the shock to his system causes an immediate dilatation of the large bloodvessels of the abdomen; this results in a great fall of blood-pressure, and the heart, finding that it has much less resistance to overcome, slackens its beats so that soon the flow of blood is very slow indeed. Hence, it has time to clot over the wound, and the man does not bleed to death. Incidentally, the feeble current of blood is insufficient to keep the most delicate organ of the body, the brain, in its normal state of activity, and the man is relieved from his pain by unconsciousness, which passes off when the heart again quickens its beat. It is perhaps needless to remark that fainting fits are not always and only caused by flesh wounds; they may be due to weakness or other causes.
Now, if we consider the instances quoted above, we are able to deduce a few general principles from them. In the first place, it may be noticed that reflex action compels us to perform the movements necessary to our existence whether we like it or no. It is not for us to decide whether we will breathe or not. We must. The strongest-willed man who ever lived, no matter how much a philosopher, could not commit suicide by holding his breath, as Cato boasted he could. Directly he lost consciousness, supposing he managed to hold out till then, the tainted blood bathing the respiratory centre would awake it to activity, and he would start breathing afresh. Again, it is noticeable that many of these actions could not possibly be performed by a voluntary effort. We can, to a certain extent, regulate the depth and frequency of our breathing, and we can blink our eyes voluntarily; but an average man would be quite at a loss what to do if asked to make the pupil of his eye dilate and contract, the glands of his stomach secrete, or his heart alter its rhythm.
It is a familiar fact that some reflex actions can be altered by an effort of the will; in other words, an impulse from a brain cell will prevent a nerve cell in the spinal cord from discharging. But it is an equally familiar fact that with continuous stimulation the impulses accumulate and ultimately overcome this resistance. Most people have at some time or other striven to resist the inclination to cough, consequent upon a tickling sensation in the throat, and know that there comes a time when they can restrain themselves no longer. This is because the accumulated stimuli from the throat, having reached a greater strength than the prohibitive impulse from the brain, succeed in compelling the cells in the cord to discharge.
Lastly, reflexes can be learnt. When a young child first endeavours to stand upright, the sensation of falling is doubtless conveyed to the brain, and thought taken of how the erect position can be maintained. But it is not until after many experiments and failures that the brain-cells can send messages to the right cells in the cord, and these set the necessary muscles in motion. Experience teaches what must be done, and constant practice eventually enables the spinal cord to act for itself without referring for orders to the brain. It is on the same principle that we learn to ride the bicycle. At first we have to devote our whole attention to keeping our balance, but in a short time we find we are doing it with our mind free to contemplate the scenery.
What can be done by reflex action can only be appreciated by observing an animal from which the brain has been removed. A frog which has been treated in this way—the operation, it should be said, if performed under an anæsthetic can cause no pain, either at the moment or afterwards—will live for weeks—in fact, almost indefinitely—if proper precautions be taken. But it is an automaton pure and simple. Unless touched it sits absolutely still. If touched it hops once or twice straight ahead regardless of obstacles. If placed in water it swims, equally regardless of obstacles. If turned on its back it immediately resumes its normal position. If small chips of wood are placed on its back it kicks them off. If the table on which it is sitting be tilted it will crawl up the incline until it reaches a level. But it will starve in the midst of plenty, having lost all power of thought, memory and perception. If diligently fed by hand a frog, a fish, or a bird will live for a long time without any brain, since their repertoire of movements is small and mostly reflex, and their occasions for deliberated action comparatively few. But the higher we get in the scale of life the more the brain takes over the duties of the cord, the less automatic become the greater number of the actions, and hence the more open does the animal’s conduct lie to moral criticism.