Sentence:  Instead , he constantly became lost in parts and components of them , confused some of their details with those of neighboring objects , and so on , unless he allowed time to `` trace '' the object in question through minute movements of the head and <WSD>hands</WSD> and in this way to discover its contours . 
Meaning of the word marked with <WSD> tags:  the (prehensile) extremity of the superior limb