The cannula is provided with a hub for mounting on a syringe. Adjacent the point of the cannula is a lateral opening which intersects the end opening. A slidable sleeve is provided for sealing to the cannula outwardly of the lateral opening during insertion of the cannula and, in some instances, during taking of the blood sample. An outwardly-extending disk is arranged at an oblique angle to the sleeve to seal against the skin of a patient.
