" But besides having this special feature which is difficult to isolate, we notice that anxiety is accompanied by fairly definite physical sensations which can be referred to particular organs of the body. [...] The clearest and most frequent ones are those connected with the respiratory organs and with the heart. They provide evidence that motor innervations—that is, processes of discharge—play a part in the general phenomenon of anxiety. "

― Sigmund Freud , Inhibitions, Symptoms and Anxiety