Tooth discolouration can lead to intrinsic or extrinsic stains . intrinsic stains may be caused by tetracycline stain , excessive fluoride intake , pulp degeneration after tooth trauma and the presence of metallic restorative materials such as silver amalgam 外在变色是由于食物中的色素附在牙面上，形成斑渍，只要洗牙便可以清除这类斑渍。内在变色的其中一个原因是在乳齿或恒齿的形成过程中，受到一些药物例如四环素的影响。
With strain , elevating a candlestick : with pain , feeling on his right temple a contused tumescence : with attention , focusing his gaze on a large dull passive and slender bright active : with solicitation , bending and downturning the upturned rugfringe : with amusement , remembering dr malachi mulligan s scheme of colour containing the gradation of green : with pleasure , repeating the words and antecedent act and perceiving through various channels of internal sensibility the consequent and concomitant tepid pleasant diffusion of gradual discolouration 他全神贯注地凝视着那庞大笨重被动的和那细溜活泼主动的，又殷勤地弯下身去，把掀起来的地毯边儿舒展成原样。他兴致勃勃地记起玛拉基穆利根博士的色彩计划，其中包括深浅有致的绿色208 。他又心怀喜悦之情重复着当时相互间的话语和动作，并通过内部种种感官，领悟着逐渐褪色所导致的温吞快感的舒散。
