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The gastrointestinal tract is a complex barrier/exchange system, and is the most important route by which macromolecules can enter the body. The main absorption takes place through villi and microvilli of the epithelium of the small and large intestines, which have an overall surface of about 200 m2. Already in 1922, it was recognized by Kumagai, that particles can translocate from the lumen of the intestinal tract via aggregation of intestinal lymphatic tissue (Peyer’s patch, containing M-cells (phagocytic enterocytes)). Uptake can also occur via the normal intestinal enterocytes. Solid particles, once in the sub-mucosal tissue, are able to enter both the lymphatic and blood circulation.

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Quite obviously mixtures of grades different from the above may be prepared by adding varying quantities of the ingredients named or of certain other ingredients in addition, such as kaolin, and then subjecting the whole to a mixing process, the higher grades of the lithopone containing the higher percentages of zinc sulfid. This has up to the present time been the actual method pursued in making commercial lithopone of different grades, although one not eminently satisfactory.

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