providing an on-line computer-accessible database and communication service in the form of an electronic directory listing various medical organizations, publications, facts, news, advertisements, public relations material, commercial and non-profit products and services, and bulletin boards along with the electronic addresses of such information and hypertext links to the electronic addresses of such information, providing on-line access to selected medical articles, lectures and publications, and providing a conduit whereby electronic messages can be transmitted to and from users of the services