The discovery of the girl's dead body has been a great shock to him. When this happens, the body has physical symptoms, such as a kind of shaking (like a kind of shivering) -"even as the fluid economy of his movements disintegrated." That is, as he walked, the usual smooth movement of his limbs was gone, and it suggests his legs felt wobbly as if they might buckle under him. His body didn't feel 'real' like he was accustomed to it feeling. He had his hand on the small of his wife's back as he walked. This means, as well as feeling these symptoms in his own body, he could feel his wife's body...and it felt solid and real. As long as he could feel that, feel his wife right there, solid and real, then it meant he was right there with her, there in the yard. He was not going to flip off into some crazy unreal world in his mind - this really had happened - they had found a dead body - it was not his crazy imagination - and the feel of his wife's body was a touchstone, a yardstick, a criterion of reality that he could use to keep himself grounded and know what was real and what was not real.
Student or Learner