Physiology

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Physiology

for the transformation of energy, and it re-
sponds to certain outside influences or stimuli.
The small mass of protoplasm known as amoe-
ba in virtue of life exhibits growth, mainte-
nance, and reproduction, and these three ac-
tivities are common to every plant and to
every animal. Animals, as a rule, have in ad-
dition the faculty of locomotion, and both the
higher plants and animals pass through a stage
of decay terminating in death.

Every living being commences life as a min-
ute mass of protoplasm, which is fundamental-
ly the same whether the organism belongs to
the animal or to the vegetable kingdom. In
the course of development the cells are differ-
entiated in diverse directions, and to varying
degrees. Some animal cells built up such a
product as bone; certain cells of the higher
plants elaborate chlorophyll, a product of pro-
toplasm, by means of which these plants are
enabled to fabricate their food out of inor-
ganic materials. Animal cells, again, for their
food require substances already organized by
pre-existing cells. In both cases the cells elab-
orate the raw food matter into more complex
substances before they assimilate it.

By the division of labor which results from
aggregation of cells a great economy of energy
is effected. Cell anabolism probably builds up
a series of bodies which have a katabolic ten-
dency—that is to say, they are liable to under-
go a splitting-up process by means of which
the molecular groups are rearranged and ener-
gy is evolved and manifested in heat and mo-
tion. A certain group of substances, some of
which are constantly present in every living
cell, is known as proteid. These proteids are
not themselves liable to spontaneous explo-
sion, but it is not unlikely that the addition
of oxygen may temporarily unite some of them
into a new compound which is readily decom-
posable. Such a view explains the necessity for
oxygen as well as the constant production of
carbon dioxide by the living cell. The sources
of the proteids of the human body are previ-
ously-formed proteids, fats, and carbohydrates
from vegetable and from animal food; and
should the materials supplied be more than
sufficient for the needs of the moment, the liv-
ing cell can store them up for future use. In
the absence of sufficient food supply the tissues
live upon themselves, the more essential per-
forming work and producing heat at the cost
of the less essential.

Alongside the muscular system as a liberator
or spender of energy must be placed the nerv-
ous tissues. Before leaving the body the nerv-

ous form of energy is wholly or almost wholly
transformed into heat. Heat and muscular
work may be regarded as practically the sole
forms in which energy leaves the mammalian
body, and nerve and muscle may be regarded
as the chief tissues by which energy is expend-
ed. All the other tissues are subservient to
these two supreme developments of proto-
plasm. The integumentary tissues clothe and
protect the muscles and nerves, and also act
as excretory organs. The respiratory system
provides the oxygen necessary for musculai
and nervous activity, while the alimentary sys-
tem, with all its accessory glands, supplies
fresh energy by the ingestion and assimilation
of food stuffs containing new stores of poten-
tial energy. The circulatory systems of blood
and lymph convey oxygen and pabulum to
these all-important tissues as well as to those
of secondary importance in the economy, and
they remove such products of katabolism as
are deleterious or of no further use to the ac-
tive cells. They also carry the waste products
to the excretory organs, whose function is the
discharge of useless or effete material formed
by the splitting up of the complex proteids.
In considering the phenomena of growth,
certain cells, the leucocytes of the blood and
the wandering connective tissue corpuscles,
may be looked upon as embryonic residues of
undifferentiated amoeboid organisms. Compar-
atively simple cells such as these grow and re-
produce their kind in the same fashion as an
amoeba. In cells more highly differentiated than
white blood and connective tissue corpuscles
reproduction is less simple and easy; but even
muscle fibres multiply by fission. Among the
higher vertebrates, nerve cells, which are tht
most highly specialized of all, lose in early
embryonic life the faculty of multiplication.
Their number is irrevocably fixed early in the
existence of an individual. But they preserve
the power of individual growth to a remark-
able extent. For the continuation of life a
process of reproduction is necessary. Through-
out the entire organic world this process con-
sists essentially in the detachment of a part of
the parent. In the higher plants and animals
reproduction is sexual, the female element un*
dergoing development only after fusion with
the male element. From the food and energy
supplied by the parent the embryo builds up
its tissues until it is fit for separate existence.
The special form which the individual ulti-
mately assumes depends upon qualities in-
herited by the embryo in the parental ele-
ments. For the physiological details of human