什么是 Internal environment？

In a multicellular animal most cells are not in contact with the outside world but bathed in an internal environment: the extracellular fluid (blood plasma, lymph, the interstitial fluid between cells) whose composition, temperature and volume the organism regulates. The cells exchange with this fluid; the fluid exchanges with the outside through the specialised surfaces. The idea is Claude Bernard’s (1865): the constancy of the internal environment is the condition of a free and independent life.