STUDIES IN THE PHYSIOLOGY OF THE NERVOUS SYSTEM. VIII. NEURAL BALANCE AND REFLEX REVERSAL, WITH A NOTE ON PROGRESSION IN THE DECEREBRATE GUINEA‐PIG

1. In the normal guinea-pig there is sometimes a dilemma of reaction in the act of the hind limb in response to mechanical stimulation of the femoral fold of skin. 2. The reaction is sometimes one of flexion and sometimes one of extension. The determining factor seems to be the assumption of an active posture. If the active posture is flexion the reaction is more likely to be flexion; if it is extension the reaction will most likely be extension. 3. Amongst the methods of procuring these active postures of peculiar interest are passive bendings of the neck. 4. This reversal of reflex action may be obtained after the complete removal of both cerebral hemispheres. It may therefore take place in the absence of cortical disturbing factors. 5. The reversal here described seems to differ in its causation from that described by Sherrington, Magnus, and v. Uexkull. To v. Uexkull's rule of reflex reversal—that the stretching of a muscle increases the likelihood that that muscle will contract in cases where a dilemma of reaction is possible—may be added another rule: a stimulus which may throw into activity either of twin antagonistic centres (and depress the activity of the other) will tend to activate that which at the time is already in a state of relatively raised activity. 6. Besides this transient reversal by means of the assumption of an active posture there is described in this paper a more permanent tilting of the central balance. This is seen after removal of the cortex on one side—as has been described in a previous paper. In general, flexion reactions are made more likely on the side of the cortical lesion, and extension reactions upon the other side. This is seen even more markedly after section of one crus cerebri—when the posture of the body becomes asymmetrical. 7. To explain these phenomena it may be supposed that one cortex, or cerebral hemisphere, exercises opposite influences on the centres situated on different sides of the body. When both hemispheres are removed, or the cortex is removed on both sides, the tilting of the central balance seems to be in the direction of extension on both sides of the body. This phenomenon may be explained by supposing that the cortical, or cerebral, influence of one side more than counterbalances the antagonistic influence from the cortex, or hemisphere, of the other side. 8. In general it may be supposed that the activity and the reflex excitability of a centre—or of a pair of antagonistic centres—go hand in hand, and that they are the resultant of innumerable excitatory and inhibitory influences which continually play upon the centre. This resultant may be termed the “neural balance.” The central balance may be tilted for a time in one direction by the temporary preponderance of one of the influences which play upon it. And it may be permanently upset by a permanent alteration in the value of the influences which compose the balance.