Just as Yealland silenced the unconscious protest of his patients by removing paralysis, the deafness, the blindness, the muteness that stood between them and the war, so, in an infinitely more gentle way, [Rivers] silenced his patients, for the stammerings, the nightmares, the tremors, the memory lapses of officers were just as unwitting protests as the grosser maladies of men. Chapter 22
Rivers sets about analysing the dream, which feels pointedly self-accusatory. The deformed man seems to him to represent Sassoon and his declaration, but he wonders about the patient in the chair. When he realises that a bridle and bit are instruments of control, however, it begins to come together. In his treatment, Rivers is exerting control over individual lives, reforming them back to soldiers. Although psychiatric treatment is meant to stop self-destructive behaviour, nothing could be more self-destructive and even suicidal then returning to war
Again the bit functions as a symbol of the control Rivers tacitly exerts by coaxing soldiers' broken minds back to a functioning level so they can return to combat to fight again. This is a thematically critical moment in the novel's portrayal of mental illness and its treatment, suggesting that if mental breakdown is the mind's natural reaction to overwhelming stress and trauma, then a psychiatrist actually does a disservice by undoing those symptoms without changing the environment that produced them. Fixing a soldier's mind and then sending him straight back to combat seems only to be muting his mind's power to protest the horror and unsustainability of war