Thoreau writes in his journal:
Miss Minott says that Dr. Spring told her that when the sap began to come up into the trees, i.e. about the middle of February (she says) then the diseases of the human body come out. The idea is that man’s body sympathizes with the rest of nature, and his pent-up humors burst forth like the sap from wounded trees. This with the mass may be that languor or other weakness commonly called spring feelings.