GROUP XX  . Digitalis.—This group-name has been given to a large number of substances which have an action similar to that of the foxglove leaves, including the active principles of strophanthus, squill, Urechites suberecta, Convallaria majalis, Nerium Oleander, Hellebores niger, Antiaris toxicaria (the upas tree), and several others . The active principles of these vary a good deal in chemical composition, but they are all non-nitrogenous neutral bodies . Their action is exerted upon muscle, and chiefly upon the muscle of the heart and blood vessels . The individual muscle-fibres con-tract and expand more perfectly, and thus the diastole and systole of the heart are rendered more complete, the pulse is slowed, and the blood-pressure is raised . The slowing of the heart is partly brought about by an action on the vagus centre . Ggour XXI . Picrotoxin.—In large doses the action of picrotoxin is exerted chiefly on the medullary nerve centres, whereby irregular tonic-clonic convulsions are produced; in minute doses it stops the secretion of sweat .