But at the termination of every reflex arc we find a final neurone, the ultimate conductive link to an effector organ (muscle or gland). This last link in the chain, e.g., the motor neurone, differs obviously in one important respect from the first link of the chain. It does not subserve exclusively impulses generated at one single receptive source, but receives impulses from many receptive sources situate in many and various regions of the body. It is the sole path which all impulses, no matter whence they come, must travel if they are to act on the muscle fibres to which it leads.
Therefore, while the receptive neurone forms a private path exclusively serving impulses of one source only, the final or efferent neurone is, so to say, a public path common to impulses arising at any of many sources of reception. A receptive field, e.g., an area of skin, is analysable into receptive points. One and the same effector organ stands in reflex connection not only with many individual points, but even with many various receptive fields. Reflexes generated in manif old sense-organs can pour their influence into one and the same muscle. Thus a limb muscle is the terminus ad quem of many reflex arcs arising in many various parts of the body. Its motor nerve is a path common to all the reflex arcs which reach that muscle.
Reflex arcs show, therefore, the general features that the initial neurone of each is a private path exclusively belonging to a single receptive point (or small group of points) ; and that finally the arcs embouch into a path leading to an effector organ ; and that their final path is common to all receptive points wheresoever they may lie in the body, so long as they have connection with the effector organ in question. Before finally converging upon the motor neurone the arcs converge to some degree. Their private paths embouch upon internuncial paths common in various de grees to groups of private paths. The terminal path may, to dis tinguish it from internuncial common paths, be called the final common path. The motor nerve to a muscle is a collection of final common paths.
Certain consequences result from this arrangement. One of these is the preclusion of essential qualitative difference between nerve-impulses arising in different afferent nerves. If two con ductors have a tract in common there can hardly be essential qualitative difference between their modes of conduction.
A second consequence is that each receptor being dependent for final communication with its effector organ upon a path not exclusively its own but common to it with certain other receptors, such nexus necessitates successive and not simultaneous use of the common path by various receptors using it to different or opposed effect.
In the simultaneous correlation of reflexes some reflexes com bine harmoniously, being reactions that mutually reinforce. These may be termed allied reflexes, and the neutral arcs which they employ allied arcs. On the other hand, some reflexes, as men tioned above, are antagonistic one to another and incompatible. These do not mutually reinforce, but stand to each other in inhibitory relation. One of them inhibits the other.
When the muscle is reflexly contracting to A with a tension 75% of that of the muscle's maximal contraction, stimulation of B is added. The contraction tension in result rises to 85% of the maximal. The inference is that for 65% of the total motor units of the muscle the excitation from A acts upon the same motor units as would that from B. In other words 65% of the total
aggregate of the motor units of the muscle are common to A and to B. The stream of impulses arriving at these common motor-units from B when A's stream is already activating them leaves their activity unchanged, failing either to increase or to diminish it. A's stream, which is engaging them, precludes B's stream from engaging them. This is termed occlusion. We may turn from the case of the group to that of the single individual unit. Suppose a to be a train of impulses reaching a central terminal of path A and impinging upon a motor-unit p, and activating it. Suppose similarly a train impinging by one of path B's terminals upon the same motor unitµ already activated by a. It has been argued that the resultant response of tc to the double series of impulses impinging on it will show interference and interruption and unsteadiness from "inhibition," the impulses generated by the one train interfering with those generated by the other owing to the refractory phase following in the wake of each impulse. Experiment fails to substantiate such unsteadiness or other evidence of "interference inhibition" even when the im pulse trains are each of them at 18o p. sec. Experiment in fact finds the reverse ; the resultant contraction being steadier and more durable under the concurrent excitation of A and B than under either alone. An important kind of observation is the following, since it shows clearly that, against the interference view above, "occlusion" is not inhibition. An afferent A activates 5o% of a given muscle's motor units. An afferent B activates a still larger percentage of the muscle's motor units. When afferent B is already activating the muscle the stimulation of A causes no outward result on the muscle, but if B's stimulus be then withdrawn A's continuing, the contraction at once falls to and remains at 5o% of maximal. A's activation was occluded by B's already existent one, but there was no inhibition. A's activation was occluded throughout the large field amounting to 5o% of the total aggregate of the motor units. Yet that not a single motor unit was inhibited, has been experimentally established by taking for B one of the relatively rare afferents which activate 00% of the muscle. Since B is then activating every motor unit it cannot be supposed that lack of A's effect when A is concurrent ly stimulated is due to inhibition of A's 5o% of the motor units; for that would suppose those motor units to be at one and the same time both active and inhibited. The mutual support that the two trains yield each other in the actually observed result argues the immediate replacement of B's effect by A's when B's lapses or intermits; and vice versa. That this would be so is borne out by other observations. Where, during the concurrence of A and B, A's effect has been occluding B from activating, on abruptly ceasing to stimulate A, so that B remains under stimulation alone, B which has till then been ineffective owing to occlusion, at once appears as effective and without hesitation or pause replaces that of A step for step as A's after-discharge subsides. The final com mon path under this competition of allied arcs is comparable with a telephone line already "busy" for one subscriber and for that time "engaged" and non-available to other subscribers. The action of the principle of the final common path may be instanced in re gard to "allied arcs" in the scratch-reflex as follows. If, while the scratch-reflex is being elicited from a skin point at the shoulder, a second point distant r omm. from the other point but also in the receptive field of skin, be stimulated, the stimulation at this second point favours the reaction from the first point. This is well seen when the stimulus at each point is of subminimal in tensity. The two stimuli, though each unable separately to invoke the reflex, yet do so when applied both at the same time. The "receptive field" of a reflex is really the common area of com mencement of a number of allied arcs.