On examining sections of such a line, it is interesting to note that at first sight the particles appear to be situated deeply in the tissue, and mainly in relation to the bloodvessels supplying the gum. A little closer attention shows that the particles are really aggregated, particularly in the deficiencies between the epithelial cells which are constantly thrown off from the surface of the gum, a process which has its origin in an inflammatory condition, the whole gum becoming hypertrophied, with numerous small areas of ulceration. In these positions a certain amount of direct absorption of dust and fine particles takes place from the ulcerated surface, and becomes converted into lead sulphide by the sulphuretted hydrogen produced locally from the decomposing tissue. A certain amount of pigmentation is also referable to the mucous glands. It is well known that, in infections of the mouth of the type of pyorrhœa alveolaris or of rarefaction of the alveolar process, a good deal of infection coexists in the mucous glands of the buccal membrane, especially along the gum margins themselves, and the lead dust also becomes deposited in these glands, and later forms a sulphide. It is possible that some blue lines are due to excretion of lead from the blood. Some little care is required to differentiate between the early lead Burtonian line and the curious bluish-grey appearance of the gum edges in cases of pyorrhœa alveolaris; when once the two conditions have been studied, little difficulty exists, but the use of a hand-lens will at once settle the matter. The bluish appearance of the gum in many cases of gum disease is due to local cyanosis. A few other forms of pigmentation of the gum edges exist, such as an occasional blue line seen in workers with mercury, a black line in coal-miners, and so on, but these hardly call for discussion in the present instance. Any pigmentation of the nature discussed above is to be regarded as a sign that the worker has been subjected to the inhalation of lead dust, and is therefore suspect of lead absorption, in whom definite symptoms of lead poisoning may be expected to occur if the exposure to the harmful influence be long-continued. 2. In the second variety of blue line the pigmentation is not confined to the gum edge or to a band rarely exceeding a millimetre in width, as is the ordinary common blue line. In this case the whole of the gingival mucous membrane from the edges of the teeth, and extending some way into the buccal sulcus, five or six millimetres or even a centimetre wide, may be seen. When this phenomenon is present, it is always associated with a marked degree of pyorrhœa alveolaris, the gums are soft, œdematous, and pus oozes from their edges, the teeth are frequently loose, and the other symptoms of disease of the os marginum are present.