The gum secreted by the silkworm which coats and protects the fibroin of silk fibre in its natural state. The silkworm has two sets of silk glands that secrete the fibroin and sericin (gum). Both these are spitted through the spinneret in its head, thereby the fibroin is coated by the sericin as it emerges. It cements the two brins to make a single filament, a bave, and aids in the construction of the cocoon. Also known simply as ‘gum’. See Brin, Bave.