Fig. 21.—The Growth and Migration of Granules of the Cerebellum.
Half a dozen nuclei of as yet undeveloped granules are seen lying beneath the pia mater. From this level to the bottom of the drawing granules are shown in successive stages of growth. These developing granules, selected from various preparations of the cortex of the cerebellum, were drawn from nature.
There are many different types of neurone. Any attempt to describe them, or to give an account of the various details of structure which recent improvements in technique have enabled anatomists to observe, would fill a lengthy treatise; and would, moreover, be beside our aim, which is limited to obtaining such an idea of the unit of the nervous system as will enable us to form a conception, however crude, of the way in which it works. From the brief account that has been given, it will be evident that anatomists are approaching to an understanding of the mechanism. It will also be evident that they have already more information than they can apply. They are cognizant of many details of structure which they cannot interpret in terms of function; and at the same time are aware of wide gaps in their knowledge regarding facts which are essential to the construction of any scheme. This much is clear: A sense-cell on the surface or beneath it is touched (probably entered) by the ultimate twig of the outer limb of a neurone whose cell-body lies in a spinal ganglion, while its inner limb, as a fibre of a posterior root, enters the spinal cord. In the spinal cord the root-fibre splits into an ascending and a descending division which rain branches into the grey matter over a considerable area above its point of entrance, and a smaller area below it. The finest twigs of these branches are to be seen in the vicinity of the cell-bodies and dendrites of certain other neurones. The axons of these second links arborize in a similar way in the vicinity of large motor cells, whose axons in turn become fibres of anterior roots. (For simplicity’s sake no reference is made to hosts of other neurones which link the ganglion-cell and the motor cell to other cells higher in the cord or brain.) An impulse generated in the sense-cell on the surface of the body runs up the root neurone into the cord, where the ultimate twigs of the posterior root-fibre offer it a wide choice of distribution. Following the path of least resistance, it passes into neurone No. 2. Again, the arborization of No. 2 offers it alternative paths. It makes a choice which lands it in No. 3. No. 3 passes the impulse on to the muscle-fibres with which it is connected. Three points are especially worthy of attention: (1) The impulse has a wide (literally, an unlimited) choice of routes. The skin of the finger is touched. Any muscle may respond, although resistance is so graded as to cause the impulse to seek in the first instance the group of muscles which is most often required to act in consequence of stimulation of the finger. This means, we may suppose, that it follows the chain which, having the smallest number of links, offers least resistance. If it cannot get through to these muscles, owing to the fact that other impulses, acting simultaneously, either increase the resistance in this particular path, blocking its way, or reduce the resistance in an alternative path, it spreads farther afield. (2) Owing to the ramification of the root-fibre which conveys it to the cord, an impulse is not limited to a single line of distribution. It reaches many secondary links. It may therefore influence various effector neurones simultaneously. For example, a stimulus which calls extensor muscles into action, at the same time inhibits their flexor antagonists. (3) The path which it finally takes is accessible to all other impulses. Its root neurone was peculiar to itself. Link No. 2 was more or less a common path. Neurone No. 3 is open to every impulse which traverses the nervous system.
Anatomy justifies the construction of the scheme just outlined. But there are many points regarding structure upon which a physiologist desires information, many details that he wants to see filled in. How is the impulse passed from the arborization of axon No. 1 to the dendrites of neurone No. 2? By what structural arrangement is resistance introduced, and how is it regulated, if it varies? Supposing the resistance to be higher in one path than in another, or supposing that more force is needed to enable an impulse to invade a wider field, how is additional energy supplied? To the first question no answer can be given at present—the mechanism by which impulses are transferred from one neurone to another is unknown; yet it is convenient to find a name for the junction of axon-endings and dendrites. It is termed a “synapse,” on the understanding that the word involves no hypothesis as to its structural nature. It is generally held that resistance is introduced into nerve-circuits at synapses; although this again is a provisional statement. The phenomena for the explanation of which the idea of synaptic resistance was introduced, may be accounted for on a purely anatomical basis of distribution. The extent to which one neurone influences another may depend upon the size of the brush of fibrils with which its axon touches it. If a certain force is needed to discharge a neurone, a nerve-current must either have a sufficiently high potential when it reaches it, or it must act upon it for a sufficient length of time. There is little to choose between the arguments which place the resistance at the synapse and those which transfer it to the nerve-cell body.
As a mechanism the nervous system is unthinkable, unless we picture its units as independent, yet capable of forming associations; as functionally discrete, yet entering into functional continuity. When acting, they act as chains. Impulses run from link to link, from the end-twigs of an axon of one cell to the dendrites of the next. Neurones are so arranged as to make it impossible for impulses to escape backwards out of dendrites into axon-twigs. In this respect the system is valved. But there is no reason for thinking of the substance of the neurone as polarized in any way. The physical accompaniment of an impulse—the electric variation—travels with equal facility up and down its axon.
There is no evidence of any specificity of neurones; on the contrary, it is clear that impulses of every kind—that is to say, from every source, for we recognize no specificity of impulses—can travel equally well through neurones of all forms. At every junction, in passing through each synapse, they are delayed. It takes at least 0·01 second (less if the knee-jerk be a true reflex action) for a message delivered to the cord by a sensory root to reach a motor root. This hundredth of a second—the sum of the delays entailed in fording two or three synapses—is regarded as the minimum reflex time. To it must be added, in considering any particular reflex action, the time taken in travelling up sensory and down motor nerves. Delay indicates resistance. If a sensory stimulus be not sufficiently pronounced to provoke a reflex action, the reflex may be obtained on intensifying it. Prolonging or repeating the stimulus—really the same thing, since sensory impulses are rhythmic, not continuous—has a far more potent effect than increasing its force. The resistance of synapses gives way after a number of impulses have bombarded them. The desire of brushing a fly from the skin, if resisted, becomes intolerably urgent after a time. A persistent outflow of impulses produced by the irritation of a spot in the cortex of the brain overwhelms the nerve-muscle system in an epileptic fit. The following is an experiment illustrating the spread of impulses from their customary path to another less often used: A piece of blotting-paper, wet with vinegar, is placed on the inner side of the thigh of a brainless frog. There is no use in trying the experiment on a frog which retains its brain; the substitution of one action for another would be an exhibition of the adaptation of means to end—a demonstration of the animal’s right of choice. Besides, the frog might choose not to act, and so the experiment would fail. The brainless frog wipes off the blotting-paper with the foot of the same side. This foot is then fixed so that the action cannot be performed, and the blotting-paper replaced. After a longer interval the frog removes it with its other foot. Evidently it is more difficult for the impulses generated by the irritation which the vinegar causes to get across the cord than it is for them to reach motor neurones on the same side. Evidently, too, the continued irritation of the vinegar adds to the travelling power of the impulses. They are strengthened until they are capable of overcoming the resistance in the longer path. “Resistance in conductors” and “potential of current” are terms with which the study of electricity has rendered us familiar; but it must be evident from the experiment just described that these terms are not really applicable to nervous phenomena, convenient though they may be for use in an allegorical sense. Holding the foot does not, by any mechanism which we can recognize, switch off the shorter circuit, yet the impulses abandon it for the longer path. There is no evidence of a struggle to free the foot that has been fixed, coincident with the spread of impulses, as they gather sufficient strength to reach the nervous mechanism of the other leg. The right foot not being available, the impulses choose the route to the left foot. Any attempt to explain this in terms of resistance and potential involves the formulation of a number of subsidiary hypotheses; easy to devise, no doubt, but stultifying to the explanation exactly in proportion as they complicate it. Yet the hypothesis of lines of greater and of less resistance (keeping as far away from electrical analogies as possible) is essential to any explanation of nervous phenomena, and is, moreover, justified by the evidence available. There are two causes in chief upon which it depends: (1) The greater the number of neurones in a linear chain, the greater is the number of synapses to be traversed. If A, B, C are in the same circuit, the sum of their resistance has to be overcome. (2) The greater the number of neurones amongst which a nerve-current has to be subdivided, the smaller the charge available for each of them. Imagine
| B | |
| A | |
| C |
so placed as to divide B and C, the charge delivered by A between. This arrangement has, probably, an anatomical expression which accounts for the relative ease or difficulty of a path, even on the supposition that impulses do not open out as they advance—do not spread along all the branches into which an axon divides—but keep to a given line. The axon of neurone A divides, to branch about B, C, and D; but its representation in the several pericellular nets (the expression may pass for the sake of the simplicity which it introduces into the picture) is unequal. In the vinegar experiment the impulses delivered to the spinal cord by the root-ganglion neurone A pass to neurone B of the posterior horn. B’s axon arborizes more freely about the cell-body of neurone C in the anterior horn of the same side than it does about neurone D in the anterior horn of the opposite side. Hence the impulses generated by the vinegar stimulate C, sufficiently to discharge it, so long as that road is open, more quickly than they stimulate D. That C should be dischargeable only so long as the foot is free implies that the activity of the neurone is in some way conditioned by its relation with the muscles which it innervates. When the foot is held this relation is interfered with, giving to the impulses generated by the continued action of the vinegar time to overcome the resistance of D.
The simile of the opening up of paths is fairly applicable to the results which follow the use of artificial stimuli. Neurones seem to link up in series under the influence of the impulses which bombard them, popping like fireworks united by a common fuse.