49$ PRINCIPLES OF GENERAL PHYSIOLOGY 



nature, as we have seen (page 425). A similar case to the latter is that of the 

 action of the sympathetic nerve on the muscles of the iris. Waymouth Reid 

 (1894) showed that, in this dilatation of the pupil, simultaneous contraction of 

 the dilator, radial muscle and inhibition of tone of the sphincter, circular 

 muscle takes place. 



The tonic contraction of muscles concerned in the maintenance of posture has 

 also been shown by Sherrington to be subject to reciprocal innervation. 



We shall see later that vasomotor and respiratory reflexes follow the 

 same law. 



DOUBLE RECIPROCAL INNERVATION 



The simple case of the antagonistic muscles of the knee joint, acted on 

 reflexly by stimulation of one whole afferent nerve, is not like the normal complex 

 state of affairs, although it shows us the elements out of which the latter is 

 constructed. Any particular motor centre is always more or less under a two- 

 fold influence of both excitation and of inhibition. This can be studied by 

 taking a pair of antagonistic muscles and two afferent nerves, one having the 

 opposite reflex effect to the other, as was done by Sherrington (1909, 2) in the 

 work already referred to. If the two nerves are excited simultaneously, the 

 effect on the movement of the joint depends on the relative strength of the two 

 stimuli. By study of the rnyograph tracings, such, as those of Fig. 156, it is 

 seen that the motor centre of each muscle is under a twofold influence ; the 

 discharge of each represents the algebraic sum of the excitatory and inhibitory 

 influences playing upon it. At a particular relative strength, both flexor and 

 extensor centres may discharge, but neither discharge is as great as it would 

 have been if the antagonistic inhibitory influence were absent (see the middle 

 tracing of Fig. 156). 



The study of this phenomenon, as Sherrington points out, shows the import- 

 ance of inhibition, not only as suppressing excitation, but as a delicate adjuster 

 of the intensity of reflex contraction, a method which is probably of frequent 

 occurrence in natural movements. 



RHYTHMIC REFLEXES 



When the intensities of the two opposing influences on the same centre are 

 nearly equally matched, a rhythmic discharge results. An indication of this is 

 seen in the middle tracing of Fig. 156. If the movements of the right and left 

 legs are observed under these conditions, flexors and extensors are seen to be 

 alternating in contraction on the two sides, so that a stepping movement results ; 

 when the one leg is flexed, the other is extended and vice versa. The 

 following explanation is suggested by Sherrington (1913, 2, p. 98): "Reflex 

 inhibition of a centre tends to superinduce in it a state of superactivity, rebound ; 

 and conversely, as has long been known, the reflex excitation of a centre tends to 

 superinduce in it a state of depressed activity, fatigue. It is therefore not 

 surprising that when the two antagonistic influences are concurrently at work on 

 a centre, and are nearly balanced, there should result a rhythmic oscillation of the 

 two ; and presumably the rate of their alternation will depend largely on the nicety 

 of balance, and on the intensity with which the processes are acting." Forbes 

 (1912, 2, p. 287) points out that if we have two opposing forces, an increasing 

 one (A) acting against a constant one (B), and if B is acting in some way to keep 

 potential energy pent up, then, as soon as A becomes greater than B, the 

 accumulated energy is released and becomes kinetic. The important point in the 

 present connection is that, when once the release of energy has begun, it proceeds 

 until more energy is released than is represented by the excess of A over B. A 

 tank into which a stream of water is flowing and provided with an outlet at the 

 bottom, closed by a spring, may serve as a rough illustration. Forbes further 

 shows that " biogen " molecules acting in accordance with the law of mass action 

 alone, would only give a continuous response to a continuous stimulus. Also that 



