f a constant figure.
Under such conditions, the level of energy production is bound to be
low, and to remain low, and the modus of its mobilization slow and
unwieldy. With thyroid is introduced the trick of _catalysis_, or the
speeding up of the vital chemical reactions, through the agency of an
_intermediate_ which accelerates the process. It is par excellence the
great catalyst of energy in the body. (A catalyst is an intermediary
like the trace of water, which will bring about an explosion between
dry oxygen and hydrogen that without it have stayed inert with the
strongest currents of electricity.) Thus it supplies a mechanism not
only for quantity output of that subtle reality we label energy, but
also an apparatus for varying the available amount of it, and for
permitting the maximum range in ease and rapidity of its utilization.
The thyroid is still another device of life for procuring more and
more variation and differentiation, its goal, as far as we can peer
through the opalescent screen upon which its manifestations quiver.

From another point of view, the thyroid may be looked upon as the
organ evolved for maintaining the same amount of iodine in the blood
as there is in sea water. Sea water was our original habitat, since,
like Venus, we have all come up out of the sea.

The more intimate study of the composition of the blood has revealed
the most astonishing parallelism between it and the compounds of sea
water. The blood is sea water, to which has been added hemoglobin as
a pigment for carrying oxygen to the cells not in direct contact with
the atmosphere, nutrients to take the place of the prey our marine
ancestors gobbled up frankly and directly, and white cells to act as
the first line of defense. To keep the concentration of iodine in the
blood a constant, the thyroid evolved, since there is no iodine in
most foods and very little in those which do contain it.

That a minimum amount of iodine in the food is necessary to health is
shown by the existence of goitre regions. Around some of the Great
Lakes in the United States, for instance, the water does not contain
enough iodine. As a result, numerous cases of goitre occur. Iodine in
the form of sodium iodide in small doses will act as a prophylactic.
The amount of iodine in the blood is about one or two parts to ten
millions, and that of the liver is about three or four parts to ten
millions. Since the liver is the most complex and active chemical
fa