Oliver Sacks in the New York Review of Books:
Nothing is more crucial to the survival and independence of organisms—be they elephants or protozoa—than the maintenance of a constant internal environment. Claude Bernard, the great French physiologist, said everything on this matter when, in the 1850s, he wrote, “La fixité du milieu intérieur est la condition de la vie libre.” Maintaining such constancy is called homeostasis. The basics of homeostasis are relatively simple but miraculously efficient at the cellular level, where ion pumps in cell membranes allow the chemical interior of cells to remain constant, whatever the vicissitudes of the external environment. More complex monitoring systems are demanded when it comes to ensuring homeostasis in multicellular organisms—animals, and human beings, in particular.
Homeostatic regulation is accomplished by the development of special nerve cells and nerve nets (plexuses) scattered throughout our bodies, as well as by direct chemical means (hormones, etc.). These scattered nerve cells and plexuses become organized into a system or confederation that is largely autonomous in its functioning; hence its name, the autonomic nervous system (ANS). The ANS was only recognized and explored in the early part of the twentieth century, whereas many of the functions of the central nervous system (CNS), especially the brain, had already been mapped in detail in the nineteenth century. This is something of a paradox, for the autonomic nervous system evolved long before the central nervous system.