A client accesses an object by issuing a request to the object. The request is an event, and it carries information including an operation, the object reference of the service provider, and actual parameters. It encompasses all of the communication infrastructure necessary to identify and locate objects, handle connection management and deliver data. It is responsible for communication of requests. That explanation clarifies why, in the analogy, we have called the ORB a nerves-center.
Stubs are a client-side invocation mechanism.
Skeletons are the server-side interface for requests. An object adapter is something that supports various styles of object implementations. It is unlikely that the ORB is a single component. It is defined by its interfaces. It delivers requests to objects and returns any output values back to clients.