613. Calculi Cancrorum.—Crabs' Stones.
SOURCE.—The stomach of the crab (Asta'cus fluviati'lis Fab. or Cancer astacus Linné), where they are formed by concretions. The crab is found in rivers throughout the North Temperate Zone.
DESCRIPTION.—The circular, plano-convex stones vary in size from 3 to 10 mm. (1/8 to 2/5 in.) in diameter, and are white and hard, changing in hot water to a rose-red; tasteless and inodorous. When treated with hydrochloric acid, they effervesce until nothing is left but a small plano-convex, cartilaginous mass.
SUBSTITUTIONS.—Artificial stones are sometimes manufactured, but can be distinguished from the true crabs' stones by treating with HCl, when, if they are artificial, they leave little or no residue.
CONSTITUENTS.—Calcium carbonate 63 per cent., calcium phosphate 17 per cent., animal matter 12 to 15 per cent., and small portions of phosphate of magnesium and sodium salts.