The definition of an organ is vague, so that listing and classifying human organs is problematic. The classical Greek definition of an organ as an instrumental part of the body has been supplemented in modern biology by stating that organs occupy a hierarchical level between systems and tissues. Not all anatomical entities on that level meet the requirement of a special function. Thus it appears that systems are composed of organs and connecting (or linking) structures, such as some connective tissues, various tubular entities, and nerves. Clin. Anat. 30:288–289, 2017. © 2017 Wiley Periodicals, Inc.