By virtue of this spinal contrast, therefore, the extension reflex predisposes to and may actually induce a flexion-reflex, and conversely the flexion-reflex predisposes to and may actually induce an extension-reflex. This process is qualified to play a part in linking reflexes together in alternating sequence.
Much of the reflex action of the limb that can be studied in the "spinal" dog bears the character of locomotion. This has been shown recently with particular clearness by the observations of Phillipson. In the stepping of the limb the flexion that raises the foot and carries it clear of the ground prepares the antagonistic arcs of extension, and, so to say, sensitizes them to respond later in their turn by the supporting and propulsive extension of the limb necessary for progression. The reflex "stepping" of the "spinal" dog proceeds without an external skin stimulus : it will continue when the dog is held in the air. The cat walks well when all four feet are anaesthetized.
A reflex movement must generate in its progress a number of further stimuli and throw up a shower of centripetal impulses from the moving muscles and joints into the spinal cord. Squeezing of muscles and stimulation of their afferent nerves and those of joints, etc., elicit reflexes. The primary reflex movement might be ex pected, therefore, of itself to initiate further reflex movement, and that secondarily to initiate further still, and so on. Yet on cessation of the external stimulus to the foot in the flexion-reflex the whole reflex comes usually at once to an end. The scratch reflex, even when violently provoked, ceases usually within two seconds of the discontinuance of the external stimulus that pro voked it. We have as yet no satisfactory explanation of this.
A reflex as it tires shows other changes besides decline in ampli tude of contraction. Thus in the flexion-reflex, the original steadi ness of the contraction decreases; it becomes tremulous, and the tremor becomes progressively more marked and more irregular.
Finally, an irregular phasic tremor of the muscles is all that re mains. It is not the flexor muscles themselves which tire out, for these, when under fatigue of the flexion-reflex contract no longer for that reflex, contract in response to the scratch-refiex which also employs them.
Similar results are furnished by the scratch-reflex, with certain differences in accord with the peculiar character of its individual charge. One of these latter is the feature that the individual beats of the scratch-reflex usually become slower and follow each other at slower frequency. Also the beats, instead of remaining fairly regular in amplitude and frequency, tend to succeed in somewhat regular groups. The beats may disappear altogether for a short time, and then for a short time reappear.
When the scratch-reflex elicited from a spot of skin is fatigued, the fatigue holds for that spot, but does not implicate the reflex as obtained from the surrounding skin. The reflex is, when tired out to stimuli at that spot, easily obtainable by stimulation two or more centimetres away.
The local fatigue of a spinal reflex seems to be recovered from with remarkable speed. A few seconds' remission of the stimulus suffices for marked though incomplete restoration of the reaction. Fatigue seems a process elaborated and preserved in the selective evolution of the neural machinery. One obvious use attaching to it is the prevention of the too prolonged continuous use of a common path by any one receptor. It precludes one receptor from occupy ing for long periods an effector organ to the exclusion of all other receptors. It prevents long continuous possession of a common path by any one reflex of considerable intensity. It favours the re ceptors taking turn about. It helps to ensure serial variety of reaction. The organism, to be successful in a million-sided environ ment, must in its reaction be many sided. Were it not for such so called fatigue, an organism might, in regard to its receptivity, de velop an eye, or an ear, or a mouth, or a hand or leg, but it would hardly develop the marvellous congeries of all those various sense organs which it is actually found to possess.