Sentence:  At these times he felt a kind of pain in his upper chest , but it was an objective pain , in no way different from others in intensity and not different in kind ; it was like the bandaged wound on the <WSD>back</WSD> of his head which occasionally throbbed ; it was merely another part of his weakness . 
Meaning of the word marked with <WSD> tags:  the side that goes last or is not normally seen