soul or spirit, that must be added to the equation that
brings us life as we know it. This was one of the central
interests of scientists and scholars during the latter part
of the 19th century, and many spent time investigating
the relationship between human beings and nature by
studying the physiology of perception, as opposed to
the mechanistic theory that all living organisms are
inanimate and mechanical. Most of the biologists of
that era believed in the existence of a life force of some
kind. It was opposed in the 19th century by meteorolo-
gist/scientist Hermann von Helmholtz (1821–94), who
spent many years trying to prove it wrong.
vitamin An essential organic nutrient that is needed
in small amounts by an organism for metabolism and
other processes. Organisms either synthesize them or
obtain them in other ways. Examples of vitamins are
vitamin C and vitamin E, both antioxidants. A vita-
min usually functions as a coenzyme or a component
of a coenzyme and is soluble in either water or organ-
ic solvent. The lack of certain vitamins can lead to
disease such as in rickets (vitamin D), tooth decay
(vitamin K), bone softening (vitamin D), or night
blindness (vitamin A). Other vitamins include vitamin
B 1 (thiamin), vitamin B 2 (riboflavin), niacinamide342 vitamin
A transmission electron micrograph of smallpox viruses. Smallpox is a serious, highly contagious, and sometimes fatal infectious dis-
ease. There is no specific treatment for smallpox, and the only prevention is vaccination.(Courtesy of Fred Murphy and Sylvia Whitfield,
Centers for Disease Control and Prevention)