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Updated: June 1, 2025
The isolation of thyroxin has made possible the determination of the influence of the thyroid hormone upon the evolution of energy in any higher animal organism. There is, for every individual, a constant, known as the metabolic rate, or the combustion rate, a reading of the rate at which his cells are consuming material for heat.
Starling and Miss Lane-Claypon concluded from their experiments on rabbits that the hormone originated in the foetuses themselves within the pregnant uterus. In virgin rabbits it is difficult to find the milk glands at all. When found the nipple is minute and sections through it show the gland to consist of only a few ducts a few millimetres in length.
In the F1, when the horned character in the female is only inherited from one side, the hereditary tendency is not enough to overcome the influence of the absence of the testis hormone and presence of the ovarian hormone, and so the horns do not develop.
A state of lability of cells at a point because of increased or decreased use. An increased or decreased appropriation by them of the hormone controlling their function. A corresponding increase or decrease in function of the gland of internal secretion and so, An increased or decreased representation of it in the reproductive sex cells in the gonads.
The fact that a hormone from the testis affects the development of the antler, as well as our knowledge of hormones in general, suggests a special theory of the heredity of somatic modifications due to external stimuli. Physiologists are apt to look for a particular gland to produce every internal secretion.
It must be admitted that the occurrence of male secondary characteristics on one side of the body, and female on the other, is in consistent with the view that the development of such characters is due to the stimulus of a hormone, since the idea of a hormone means something which diffuses by way of the blood-vessels, lymph-vessels, and interstices of the tissues, throughout the body, and the hormone theory of secondary sexual characters assumes that these characters are potentially present by heredity in both sexes.
The development, therefore, may be considered to be related to the presence of a hormone derived from the normal testis, but not to a special quantity or quality of hormone associated with maturity or the functional activity of the organ. In Rodents, however, there is a difference in the organs, not only at maturity, but in every rutting season, at any rate in Muridae such as rats and others.
The facts agree with the hormone theory, for it is to be noted that although the development of the scrotum is confined to the males, the 'descent' or dislocation takes place in the foetus, and not at the period of puberty.
The hormone in the case of the pregnant rabbit is of course acting continuously for the whole period of pregnancy, while the artificial injection took place only once in twenty-four hours, and the amount of hormone it contained may have been absorbed in a very short time. The amount of growth obtained experimentally in five weeks was less than that occurring in pregnancy in nine days.
What we know of the endocrine factors in heredity leads us to suppose that it is the mating of one marked endocrine insufficiency with another that is often responsible for the inherited tendency to feeble-mindedness and insanity. The effect of the hormone system upon the vegetative apparatus may create the more obscure insanities and quasi-insanities.
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