It might be of interest if we were to review
briefly some of the most important lessons that
may be learned from a study of the history of the
gland.
Possibly the first of these is the
extreme complexity of the endocrine system. One
gland, for example secretes into the blood-stream
small quantities of a substance which modifies
the activities of a second gland, which, in its
turn, is able to influence yet another organ;
moreover it appears that the intermediary
gland is able to influence the secretion of the
hormone by which it is itself stimulated. Thus
Crew and Mirskava, in a recent paper, write:
"In the case of the normal immature mouse,
experiment has shown that the physiological
activity of the anterior pituitary invokes
ovarian activity which leads to mating,
and which, in turn provokes further
pituitary activity, which incites further
ovarian activity which exerts the stimulus
to uterine development."
The endocrine system appears to form what
has been called, "a closed system of interrelated
factors" and its further analysis will prove to be a most
formidable task.
Claude Bernard was the first to recognise
the physiological importance of the internal
environment which the animal created from the
products of its own metabolic processes; he showed how, in the course of evolutionary time, 'le milieu organique
se specifie et s'isole en quelque sort de plus en
plus du milieu ambiant"; and now from a study. of
the pituitary body and of organs like it, we
may understand how, once the pH.,osmotic tension, salt-balance &c., of the body-fluids had
become approximately constant, it became possible for an animal to construct complex and accurate methods of integrating its functions by the use of infinitesimal quantities 'of specific chemical substances.
In Invertebrates where the means of maintaining
even physical and chemical conditions in
the body -fluids are primitive and unsuccessful,
systems of endocrine organs rivalling in complexity
those of the vertebrates have never been discovered,
and it is noteworthy that the hormone which is
most widely distributed in these animals appears
to be adrenalin, which we now consider to be
the basis of a certain type of muscular activity
and not necessarily a true hormone at all.
We may notice for example that the
secondary sexual characters of arthropods have
been found by Meisenheimer and others to be quite independent of their reproductive organs, and that C.M. Child
has recently met with striking success in his
attempts to explain the physiology of morphogenisis
in Planaria on the basis of a simple kind of
nervous conduction.
In these forms each cell must be
regarded as differentiating under the direct
control of its own genotype, and of the few
relatively constant features of its environment
(among which we may reckon the "metabolic gradient" of Child and his school).
With the introduction of organs like the pituitary
body the cell comes to be affected more and more
by the character fluid in its neighbourhood, and the number of possible phenotypes is strikingly increased since the body-fluids may, even in mammals, be modified
experimentally, or by normal environmental
factors. Consider the case of two genetically
identical axolotls; the stimulus to metamorphosis
is not evidently a genetic factor since either
animal will metamorphose when exhibited to certain
conditions. The same is true of milk-production
in cows and of fertility in almost any animal.
One can easily realise that such an
intricate and perfect system of humoral integration
may well endow the mammalian organism with a
degree of functional plasticity, which by
enabling it to respond more rapidly and accurately
to the changes of a fluctuating environment,
would heighten its biological efficiency and
increase the chances of its survival. A lower
and simpler animal might, in similar circumstances
require to wait for many centuries before a
suitable gene-mutation offered itself, and in
the event of the environmental factor reverting
to its original condition, would find itself
at a considerable disadvantage.
We have now come to the end of
our survey, and the reader might be advised, at
this point to turn back and glance at the passage
written exactly half a century ago in which
Balfour declares that, although the pituitary
body was probably employed by the early
Chordates to increase the delicacy of their
perceptions. It is more than likely that', if
the present rate of progress continue , our
own theories will, in fifty years time be
considered almost as inadequate as Balfour's
are today.
