Class you want not listed? Take it independently!
You can register for practically any Cornell class if you can find a professor willing to supervise the coursework through independent study. (This may be an actual independent study class or it may be any other course that a faculty member is willing to offer on a tutorial basis. You can find class lists in Cornell's Courses of Study.)
Once you have identified a professor willing to supervise your study, please follow the steps outlined in the section "Students wanting to register for independent study" in the registration pages of this site. This must be done at least two weeks prior to beginning the independent study class.
In addition, a group of students (at least five) wishing to enroll in a specific class not scheduled in their chosen session may write and sign a letter or email and send it to the School of Continuing Education and Summer Sessions registrar (B20 Day Hall Ithaca, NY 14853, or email@example.com) requesting that the class be added to the roster. The registrar's office will then try to arrange for that course to be offered.