.--CAPACITY OF ADAPTATION
TO THE ENVIRONMENT SHOWN BY LIVING MATTER--INDIVIDUALITY OF LIVING
MATTER.--THE CAUSES OF DISEASE.--EXTRINSIC.--THE RELATION OF THE HUMAN
BODY TO THE ENVIRONMENT.--THE SURFACES OF THE BODY.--THE INCREASE OF
SURFACE BY GLAND FORMATION.--THE REAL INTERIOR OF THE BODY REPRESENTED
BY THE VARIOUS STRUCTURES PLACED BETWEEN THE SURFACES.--THE FLUIDS OF
THE BODY.--THE NERVOUS SYSTEM.--THE HEART AND BLOOD-VESSELS.--THE
CELLS OF THE BLOOD.--THE DUCTLESS GLANDS.

There is great difficulty, in the case of a subject so large and
complex as is disease, in giving a definition which will be accurate
and comprehensive. Disease may be defined as "A change produced in
living things in consequence of which they are no longer in harmony
with their environment." It is evident that this conception of disease
is inseparable from the idea of life, since only a living thing can
become diseased. In any dead body there has been a preexisting disease
or injury, and, in consequence of the change produced, that particular
form of activity which constitutes life has ceased. Changes such as
putrefaction take place in the dead body, but they are changes which
would take place in any mass similarly constituted, and are not
influenced by the fact that the mass was once living. Disease may also
be thought of as the negation of the normal. There is, however, in
living things no definite type for the normal. An ideal normal type
may be constructed by taking the average of a large number of
individuals; but any single individual of the group will, to a greater
or less extent, depart from it. No two individuals have been found in
whom all the Bertillon measurements agree. Disease has reference to
the individual; conditions which in one individual would be regarded
as disease need not be so regarded in another. Comparisons between
health and disease, the normal and the abnormal, must be made not
between the ideal normal and abnormal, but between what constitutes
the normal or usual and the abnormal in a particular individual.

The conception of disease is so inseparably associated with that of
life that a brief review of the structure and properties of living
things is necessary for the comprehension of the definition which has
been given. Living matter is subject to the laws which govern matter,
and like matter of any other sort it is composed of atoms and
molecules. There is no force inherent in living matter, no vital force
inde