The Doctor in front of a Vulcan reproductive gland
The Vulcan reproductive gland was a part of the Vulcan body that had, as the name implies, an important function in Vulcan reproduction.
In 2375 The Doctor hosted a slideshow of some of his pictures on the holodeck. Among those pictures was a huge image of the Vulcan reproductive gland. The Doctor marveled at its composition and the subtle variations in color. (VOY: "Nothing Human")