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History of our knowledge regarding the functions of the pituitary gland

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McWhanJ_1931redux.pdf (24.14Mb)
Date
1931
Author
McWhan, John
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Abstract
 
 
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.
 
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http://hdl.handle.net/1842/35297
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