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dinated chemical and mechanical responses to both
external and internal conditions. Therefore, every part
of the body needs to be in close contact with the rapid-
fire tentacles of the nervous system.
The functional unit of this system is the single neuron,
and its physiological center is clearly the largest and
densest plexus of neurons within it - the brain.
Although the concept can clearly be seen in the early
attempt by Vesalius, notice that in his conception the
veins and arteries do not join with each other - it would
take another two centuries for William Harvey to dis-
cover capillaries and the closed nature of the circulatory
net. A full accounting would show tens of thousands of
miles (about 100000 km) of capillary nets, giving us
another filmy 'vascular body' that would be complete
down to the finest detail (Figs 1.20-1.22 or see the com-
plete system modeled at If we
included the lymphatic and the cerebrospinal fluid cir-
culation in our consideration of the vascular system, our
'fluid human' would be even more complete, down to
the finest nuances of everything except hair and some
gaps created by the avascular parts of cartilage and
dense bone.
In any multicellular organism - and especially true
for those who have crawled out onto dry land - the
inner cells, which are not in direct communication with
the outside world, depend on the vascular system to
bring nourishing chemistry from the edge of the organ-
The fluid net
Similarly, if we made everything invisible but the vas-
cular system, we would once again have a filmy repre-
sentation that would show us the exact shape of the
body in question (Fig. 1.19). Centered around the heart's
incessant pump, its major arteries and veins go to and
from the lungs, and out through the aorta and arteries
to the organs and every part of the body via the wide
network of capillaries.
Fig. 1.20 A cast of the venous system inside the liver from below.
The sac in the center is the gall bladder. (© Ralph T Hutchings.
Reproduced from Abrahams et al 1998.)
Fig. 1.19 Vesalius, in 1548, also created a picture of our second
whole-body system, the circulatory system. (Reproduced with
permission from Saunders JB, O'Malley C. Dover Publications;
Fig. 1.21 Even with just these few large arteries represented, we
can see something about this person. You might guess a Nilo-
Hamitic person, for instance, but it is, in fact, an infant. (© Ralph T
Hutchings. Reproduced from Abrahams et al 1998.)
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