Rivers discusses the peculiarity of an officer suffering from mutism, which is a physical symptom commonly found in private soldiers along with paralysis, blindness, deafness, explaining that stammering is much more frequent in officers. Mutism seems to spring form a conflict between wanting to say something, and knowing that if you do say it the consequences will be disastrous. So you resolve it by making it physically impossible for yourself to speak. Rivers explains that for a private soldier the consequences would be much worse that for an officer.
Barker then uses mutism in order to show the class difference of opinions; that the upper classes could except and validate psychosomatic disorders whereas for the working class the illness has to be physical in order to be taken seriously. What has to be remembered is that Prior does come from a working class family. Even Prior himself believes the memory loss he suffers from is brought on by the conflict of needing to remember something and, at the same time, the desire to forget it. All he remembered was that two of his men were killed and that maybe he had been responsible for it.