Sentence:  He was seeing , somehow , the face of a young boy , the boy Simms Purdew must once have been , a boy with sorrel hair , and blue eyes dancing with gaiety , and the boy <WSD>mouth</WSD> grinning trustfully among the freckles . 
Meaning of the word marked with <WSD> tags:  the externally visible part of the oral cavity on the face and the system of organs surrounding the opening