Internal and External Environment—Le Milieu
Intérieur. Human history during the past two hundred
years provides striking examples of man's desire and
ability to change the trend of his relationships with
environmental forces.
Throughout the nineteenth century, one of the
manifestations of the Hippocratic doctrine, according
to which man can escape disease by living reasonably,
was the interest that prevailed throughout Europe in
the health and ways of life of the noble savage. The
illusion (and it still persists!) was that civilized man
could recapture biological and mental sanity by re
turning to the ways of primitive life in nature. Naive
as it was, and contradicted by epidemiological facts,
this attitude was of practical importance nevertheless
because it created an intellectual climate favorable for
the Sanitary Revolution of the mid-nineteenth century.
If ever men lived under environmental conditions
completely removed from the ways of nature, and
deleterious to health in all ways, it was the proletariat
of the first phase of the Industrial Revolution. Physi-
cians and enlightened citizens could not help noticing
that biological and mental degeneration always ac-
companied dirt, want, and other aspects of the appall-
ing conditions that prevailed in the factories, shops,
and tenements of industrial cities. This awareness led
to the conclusion that it was a social responsibility to
provide the multitudes with pure air, pure water, pure
food, and pleasant surroundings. In Munich, the chem-
ist Max von Pettenkoffer, who was city administrator,
practiced with astonishing success the policies of sani-
tation that he formulated later in his pamphlet, “The
Value of Health to a City,” consisting of two popular
lectures delivered on March 26 and 29, 1873. For him,
collective cleanliness was the surest way to health and
happiness. The sanitary movement in England gener-
ated the “Health and Town Association,” which took
as a goal “to bring home to the poorest... the simple
blessings which ignorance and negligence have long
combined to limit or spoil: Air, Water, Light.” In 1876,
the English physician Benjamin Ward Richardson pub-
lished his book Hygeia: A City of Health describing a
utopia based on what he regarded as rational control
of the environment by social regulations.
The publication of von Pettenkoffer's “The Value
of Health to a City” and of Richardson's Hygeia marks
the height of the nineteenth-century sanitary revolu-
tion. But, paradoxically enough, it also marks its end
because, as these books were being published, the
environmental philosophy that they advocated was
beginning to be overshadowed by the more precise
environmental knowledge derived from laboratory sci-
ence. As a result of the discoveries made in the 1870's,
especially by bacteriologists and nutritionists, the
cleansing of the environment rapidly lost the limelight
to chemical disinfection, vaccination, drugs, and vita-
mins. Charles V. Chapin, the Health Commissioner of
Providence, Rhode Island, symbolized this change of
attitude by stating that it mattered little from the point
of view of hygiene whether city streets were clean or
not, provided microbes were kept under control and
people were protected against infection by the proper
vaccines. Environmental control of health was being
replaced by laboratory control.
The laboratory scientists, who superseded the nine-
teenth-century sanitarians on the health scene, worked
on the assumption that each particular disease could
be equated with the effect of a specific causative
agent—whether this be a microbial pathogen, a nutri-
tional deficiency, a metabolic defect, or a mental stress.
The evidence in favor of the doctrine of specific etiol-
ogy is so impressive indeed that it appears to leave
no place for the rather vague Hippocratic hypothesis
according to which disease results from the breakdown
of the harmonious equilibrium that exists in the state
of health between organism and environment. Hippo-
cratic doctrine has acquired a new and more profound
significance during recent years from the broad impli-
cations of the concepts identified with the names of
Charles Darwin and Claude Bernard. Interestingly
enough, these new concepts were first being formulated
a century ago, precisely at the time when the doctrine
of specific etiology was beginning to gain momentum.
Darwinian evolution through natural selection im-
plies that an organism cannot be biologically successful
unless it is well adapted to its external environment.
Genetic science has defined furthermore how this
adaptation is achieved through selective processes
which are under the control of environmental forces.
As is now well understood, mutation and selection
provide the mechanisms which make for adaptation
to the environment and which progressively become
incorporated in the genetic apparatus of the species.
While mutation and natural selection account for
the evolutionary evolution of species, these processes
contribute little to the understanding of the precise
mechanisms through which each individual organism
responds adaptively to its environment. This comple-
mentary knowledge has evolved in large part from
Claude Bernard's visionary concepts regarding the
interplay between the external environment and what
he called the internal environment. Claude Bernard
formulated the hypothesis that organisms could not
maintain their individuality and could not survive if
they did not have mechanisms enabling them to resist
the impact of the outside world. Whether man, animal,
plant, or microbe, the organism can function only if
its internal environment remains stable, at least within
narrow limits. The constancy of the internal environ-
ment determines in fact the organism's individuality.
In the case of man, it involves not only biological
attributes but also mental characteristics.
The recognition that the internal environment must
remain essentially stable, even when the external envi-
ronment fluctuates widely, constitutes such an impor-
tant landmark in biological thought that Claude
Bernard's phrase “milieu intérieur” has gained accept-
ance in the English language. The most commonly
quoted expression of his law is: “The constancy of the
milieu intérieur is the essential condition of inde
pendent life.” But its most complete expression occurs
in the Leçons sur les phénomènes de la vie communs
aux animaux et aux végétaux (1878-79):
The fixity of the milieu intérieur supposes a perfection of
the organism such that the external variations are at each
instant compensated for and equilibrated. Therefore, far
from being indifferent to the external world, the higher
animal is on the contrary constrained in a close and master-
ful [savante] relation with it, of such fashion that its equi-
librium results from a continuous and delicate compensation
established as if by the most sensitive of balances.... All
of the vital mechanisms, however varied they may be, have
always but one goal, to maintain the uniformity of the
conditions of life in the internal environment
(Holmes, in
Grande and Visscher, p. 188).
William James was one of the first to recognize the
philosophical importance of the milieu intérieur con-
cept and he referred to it in an editorial in the North
American Review as early as 1868. But it was probably
Lawrence J. Henderson who did most to make scien-
tists aware of the concept by discussing it in the intro-
ductory chapter of his book Blood, A Study in General
Physiology (New Haven, 1928).
