In medicine, the word capsule is used for a small gelatinous case enclosing a dose of medication.
Capsules, meaning a metal cap or cover on bottles and jars as on wine bottles, were historically made of lead, and protected the cork from being gnawed away by rodents or infested with cork weevil. Because of research showing that trace amounts of lead could remain on the lip of the bottle, lead capsules were slowly phased out, and by the 1990s, most capsules were made of aluminum foil or plastic.
In anatomy, the term capsule is used to denote a cover or envelope partly or wholly surrounding a structure. Every diarthrodial joint possesses a fibrous or ligamentous capsule, lined with synovial membrane, attached to the adjacent ends of the articulating bones. The term is particularly applied to the sac which encloses the crystalline lens of the eye; to Glisson's capsule, a thin areolar coat of fibrous tissue lying inside the tunica serosa of the liver; to the glomerular capsules in the kidney substance; to the suprarenal capsules, two small flattened organs in the epigastric region; and to the internal and external capsules of the brain.
