158 INTELLIGENT DEVELOPMENTassembly. But their activities then need to be coordinated so that the
whole functions harmoniously in the ever- changing environment. As
with development, that coordination is achieved through signaling sys-
tems among cells that are also sensitive to environmental changes outside
the body. Th is is what is called “physiology.” Th e nature of physiology, and
how that coordination occurs, tells us more about the nature of potential
and its variation.
Physiology is based on communication, a crucial obligation of cells
and tissues in multicellular organisms. One cell separated from the rest
in a culture dish soon dies without the usual storm of signals from others,
even when provided with all the nutrients and other conditions it needs.
Very early in evolution, then, a wide range of chemical messenger systems
(including pheromones, prostaglandins, interferons, and hormones) be-
came the crucial aspect of the physiology of organisms.
Th e most prominent and best known of these forms the endocrine
system: a collection of glands that produce hormones that infl uence
many functions of the body. Released into the circulating body fl uids
(or bloodstream in more complex species), they fi nd their way to specifi c
target cells and inform them about what to do. More than fi ft y human
hormones have been identifi ed, all acting by binding to target cell re-
ceptors, then fi ring up the internal signaling systems as described in
chapter 4.
It used to be thought that physiology performs an essential
equilibrium or homeostatic function, refl ecting idealistic views of the
environment as essentially stable or cyclically recurring. In that view,
each physiological function in de pen dently maintains, as far as pos si ble,
some constancy of part of the internal milieu— blood sugar, tempera-
ture, salt balance, or whatever—in the face of disturbances from inside
or outside. As Steven Rose pointed out in his book, Lifelines, “No modern
textbook account of physiological or psychological mechanisms fails to
locate itself within this homeostatic meta phor.”^17
A good example would be body temperature regulation. To be sure,
regulation includes a thermostat- like center located in the hypothala-
mus at the base of the brain. Nerve receptors in the skin and spinal cord
provide feedback about changes to outside and inside temperatures.
Th e body is then driven to either conserve heat (piloerection, or hairs
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