It would seem a general rule that reflexes arising in species of receptors which considered as sense-organs provoke strongly af fective sensation caeteris paribus prevail over reflexes of other species when in competition with them for the use of the "final common path." Of all reflexes it is those of ordinary posture that are the most easily interrupted by other reflexes. Even a weak stimulation of the noci-ceptive arcs arising in the foot often suffices to lower or abolish the knee-jerk or the reflex extensor tonus of the elbow or knee. If various species of reflex are arranged, therefore, in their order of potency in regard to power to interrupt one another, the reflexes initiated in receptors which considered as sense-organs excite sensations of strong affective quality lie at the upper end of the scale, and the reflexes that are answerable for the postural tones of skeletal muscles lie at the lower end of the scale. One great function of the tonic reflexes is to maintain habitual attitudes and postures. They form, therefore, a nervous background of active equilibrium. It is of obvious advantage that this equi librium should be easily upset, so that the animal may respond agilely to the passing events that break upon it as intercurrent stimuli.
Results.—Intensity of stimulation, fatigue and freshness, spinal induction, functional species of reflex, are all, therefore, physio logical factors influencing the result of the interaction of reflex arcs at a common path. It is noticeable that they all resolve them selves ultimately into intensity of reaction. Thus, intensity of stimulus means as a rule intensity of reaction. Those species of reflex which are habitually prepotent in interaction with others are those which are habitually intense those specially impotent in competition are those habitually feeble in intensity, e.g., skeletal muscular tone. The tonic reflexes of attitude are of habitually low intensity, easily interfered with and temporarily suppressed by intercurrent reflexes, these latter having higher intensity.
The high variability of reflex reactions from experiment to ex periment, and from observation to observation, is admittedly one of the difficulties that has retarded knowledge of them. Their vari ability, though often attributed to general conditions of nutrition, or to local blood-supply, etc., seems far more often due to changes produced in the central nervous organ by its own functional con ductive activity apart from fatigue. This functional activity itself causes from moment to moment the temporary opening of some connections and the closure of others. The chains of neu rones, the conductive lines, have been, especially in recent years, by the methods of Golgi, Ehrlich, Apathy, Cajal and others, richly revealed to the microscope. Anatomical tracing of these may be likened, though more difficult to accomplish, to tracing the distribution of blood vessels after Harvey's discovery had given them meaning, but before the vasomotor mechanism was discovered. The blood vessels of an organ may be turgid at
one time, constricted almost to obliteration at another. With the conductive network of the nervous system the temporal variations are even greater, for they extend to absolute withdrawal of nerv ous influence. Under reflex inhibition a skeletal muscle is relaxed, to its post-mortem length, i.e., there may then be no longer evi dence of even a tonic influence on it by its motor neurone. The final common path is handed from some group of a plus class of afferent arcs to some group of a minus class, or of a rhythmic class, and then back to one of the previous groups again, and so on. The conductive web changes its functional pattern with certain limits to and fro. It changes its pattern at the entrances to com mon paths. The changes in its pattern occur there in virtue of interaction between rival reflexes : occlusion, substitution by equi valence, inhibition, immediate induction, successive induction, fatigue, are factors. As a tap to a kaleidoscope, so a new stimulus that strikes the receptive surfaces causes in the central organ a shift of functional pattern of the linkage. The central organ is a vast network whose lines of conduction follow a certain scheme of pattern, but within that pattern the details of connection are, at the entrance to each common path, mutable. The grey matter may be compared with a telephone exchange, where, from moment to moment, though the end-points of the system are fixed, the con nections between starting-points and terminal points are changed to suit passing requirements, as the functional points are shifted at a great railway junction. In order to realize the exchange at work, one must add to its purely spatial plan the temporal datum that within certain limits the connections of the lines shift to and fro from minute to minute. An example is the "reciprocal innerva tion" of antagonistic muscles—when one muscle of the antago nistic couple is thrown into action the other is thrown out of action. This is only a widely spread case of the general rule that antago nistic reflexes interfere where they embouch upon the same final common paths. And that general rule is part of the general prin ciple of the mutual interaction of reflexes that impinge upon the same common path. Unlike reflexes have successive but not simul taneous use of the common path; like reflexes mutually reinforce each other on their common path. Expressed teleologically, the common path, although economically subservient for many and various purposes, is adapted to serve but one purpose at a time. Hence it is a co-ordinating mechanism and prevents confusion by restricting the use of the organ, its minister, to but one action at a time.