A horse's bit. Not an electrode, not a teaspoon. A bit. An instrument of control. Obviously he and Yealland were both in the business of controlling people. Each of them fitted young men back into the role of warrior, a role they had—however unconsciously—rejected. He found himself wondering once or twice recently what possible meaning the restoration of mental health could have in relation to his work.
These lines are Rivers's thoughts in Part Four, Chapter 22, of the novel. After witnessing Yealland's treatment of his patients with electro-shock therapy, Rivers has a nightmare in which he is a doctor trying to shove something into the mouth of a patient who resists him. After he wakes, he realizes that what he was trying to shove into the mouth was a horse's bit, and that the patient who resisted him in his dream was Sassoon. This passage evidences Rivers's thought process as he works out the meaning of his dream. He reasons that the bit, an instrument of control, must symbolize the control he has over his patients—how he forces them into roles they no longer desire.
Throughout the passage, Barker skillfully employs short, incomplete sentences to mimic Rivers's train of thought. These sentences draw emphasis to the bit, which turns out to be the key to understanding the dream. Rivers's nightmare underscores the theme of control in the novel, and forces us to consider the difference between the methods of Rivers and Yealland. The nightmare raises the question of whether the method of control matters if the end is the same. As an arm of the state, a society whose values Rivers now questions, he is forced to reconsider his role as a healer to his patients.