Biology Reference
In-Depth Information
The key to Bernard's response was to focus on the internal organization of living
systems and to argue that the internal parts of a living mechanism resided in an
internal environment that is distinct from the external environment in which the
organism as a whole dwells. This provided a ready account of the indeterminacy
exhibited by living mechanisms. Whereas there might not be strict determinism
in the response of a part of an organism to changes in the external environ-
ment, he maintained that strict determinism could be found in its response to
conditions of the internal environment. For example, whereas fluctuations in the
sugar available in food might not lead to changed metabolic activity in somatic
tissue, decreased glucose levels in the blood would result in lowered metabolic
activity in somatic tissue. The focus on the internal environment also provided
Bernard the beginnings of a response to Bichat's contention that organisms
are not mechanistic insofar as they operate to resist physical processes in their
environment. The internal environment provides a buffer between conditions
in the external environment and the reactive components of the mechanism,
insulating component parts of the mechanism from conditions in the external
environment. Bernard proposed that this buffering is achieved by individual
components of the organism, each performing specific operations that served
to maintain the constancy of the internal environment.
12
Insofar as some of its
mechanisms are designed to maintain a constant internal environment despite
changes in the external environment, a living system can appear as an active
system doing things that resist its own demolition.
13
Although emphasizing the role of organs in the body in maintaining the con-
stancy of the internal environment, Bernard did not provide a detailed account of
how organs might operate in this way. Walter Cannon (1929) introduced the term
homeostasis (from the Greek words for
same
and
state
) for the capacity of living
systems to maintain a relatively constant internal environment. He also sketched
a taxonomy of strategies through which animals are capable of maintaining
homeostasis. The simplest involves storing surplus supplies in time of plenty,
either by simple accumulation in selected tissues (e.g., water in muscle or skin)
or by conversion to a different form (e.g., glucose into glycogen) from which
reconversion in time of need is possible. A second kind of homeostasis involves
altering the rate of continuous processes (e.g., changing the rate of blood flow
by modifying the size of capillaries to maintain uniform temperature). Cannon
noted such control mechanisms are regulated by the autonomic nervous system.
12
Bernard, for example, says 'all the vital mechanisms, however varied they may be, have only one object,
that of preserving constant the conditions of life in the internal environment' (Bernard, 1878, p. 121, translated
in Cannon, 1929, p. 400).
13
Bernard's focus seems to have been more on the constancy of the internal environment than on just what
the conditions in the internal environment were. A key feature of living systems is that via control of flow of
materials across membranes they create environments different from those found outside them and in these
internal environments component parts operate differently than they do in the external world.
