II. PNEUMA
The notion of a mysterious extra element in the
living body began to appear
in Aristotle's time, espe-
cially among those
inclined towards a materialistic
explanation. Aristotle believed in a
special fifth ele-
ment, the aether, but he confined it to the heavens.
In biology he uses a
concept of innate breath (pneuma),
which is mingled
with vital heat and is present even
in nonbreathing animals. It transmits
sensation, and is
the conveyor of soul in the semen. At one point he
compares its generative power with the aether, but
otherwise he defines it as warmed air, which the body
replenishes by
respiration.
After Aristotle the pneuma concept spread widely,
with different applications according to the different
philosophical
positions. Among medical writers it was
the residuary legatee of
unattributed functions, psychic
and sensory. The Stoics equated it with the divine
Logos that permeates nature, identifying pneuma and
aether (which remain confused in the medieval
concept
of the fifth element or quinta
essentia). Even the atom-
ists
posited a special kind of atom to account for the
soul, which Epicurus said
consists of atoms of fire, air,
wind, and a nameless fourth
kind—thus reducing his
opponents' pneuma
to wind but replacing its mysteri-
ous
functions with a new mystery.
Aristotle's predecessors had debated whether the
brain or the heart is the
center of sensation, and
whether the heart or the liver is the source of
the blood.
He decided for the heart in both cases. But in the third
century B.C. the Alexandrian anatomists distinguished
arteries from veins,
and isolated the nervous system
with the brain as its base. This led to an
elaboration
of pneuma theory: venous blood is formed
in the liver,
where it is charged with the lowest grade of pneuma
(Latin spiritus
naturalis, “natural spirits” in Renais-
sance medicine); some venous blood
flows to the heart,
where it is mixed with “pneuma necessary for life”
(“vital
spirits”) and so becomes arterial blood; this flows
to the
brain, where its pneuma is purified into “pneuma
used by the soul” (spiritus animalis, “animal
spirits”).
To explain the blood's route, Galen in the second
cen-
tury A.D. postulated minute passages
leading from right
to left ventricle in the heart, for which later
anatomists
searched in vain. After Harvey had demonstrated the
circulation of the blood, the whole hypothesis of
pneuma was gradually discarded.
